Degree Of Protandry Research Articles

Seasonal and daily protandry in a cyprinid fish

In polygynandrous mating systems, in which females limit reproductive success, males can increase their success by investing in courtship. Earlier arrival at the spawning ground compared to when females arrive may increase their opportunities in competitive mating systems. In this study, we used passive telemetry to test whether a male minnow known as the asp, Leuciscus aspius, times its arrival at spawning grounds relative to the arrival of females. Males arrived in a model stream approximately five days earlier than females on average and left four to five days later than females over two years. Both sexes performed a daily migration between a staging ground (standing water, low energy costs) and the fluvial spawning ground (high energy costs). Fish abundance peaked twice a day, with a major peak at sunset and a minor peak at sunrise and with the evening peak abundance for males occurring 1 hour 40 minutes earlier than that of females. The number of females on the spawning ground never exceeded the number of males. While the degree of protandry is hypothesized to be influenced by the operational sex ratio (ranging from 0.5 to 1 in our study), our data did not support this theory.

Long‐term phenological shifts and intra‐specific differences in migratory change in the willow warblerPhylloscopus trochilus

Climate change can influence many aspects of avian phenology and especially migratory shifts and changes in breeding onset receive much research interest in this context. However, changes in these different life‐cycle events in birds are often investigated separately and by means of ringing records of mixed populations. In this long‐term study on the willow warblerPhylloscopus trochilus, we investigated timing of spring and autumn migration in conjunction with timing of breeding. We made distinction among individuals with regard to age, sex, juvenile origin and migratory phase. The data set comprised 22‐yr of ringing records and two temporally separated data sets of egg‐laying dates and arrival of the breeding population close to the ringing site. The results reveal an overall advancement consistent in most, but not all, phenological events. During spring migration, early and median passage of males and females became earlier by between 4.4 to 6.3 d and median egg‐laying dates became earlier by 5 d. Male arrival advanced more, which may lead to an increase in the degree of protandry in the future. Among breeding individuals, only female arrival advanced in timing. In autumn, adults and locally hatched juvenile females did not advanced median passage, but locally hatched juvenile males appeared 4.2 d earlier. Migrating juvenile males and females advanced passage both in early and median migratory phase by between 8.4 to 10.1 d. The dissimilarities in the response between birds of different age, sex and migratory phase emphasize that environmental change may elicit intra‐specific selection pressures. The overall consistency of the phenological change in spring, autumn and egg‐laying, coupled with the unchanged number of days between median spring and autumn migration in adults, indicate that the breeding area residence has advanced seasonally but remained temporally constant.

Effects of climate change on the degree of protandry in migratory songbirds

CR Climate Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials CR 35:107-114 (2007) - DOI: https://doi.org/10.3354/cr00717 Effects of climate change on the degree of protandry in migratory songbirds Kalle Rainio1, Anders P. Tøttrup2, Esa Lehikoinen1, Timothy Coppack3,* 1Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland 2Center for Macroecology, Institute of Biology, University of Copenhagen, Universitetsparken 15, 2100 Copenhagen Ø, Denmark 3Institute of Avian Research, An der Vogelwarte 21, 26386 Wilhelmshaven, Germany *-Corresponding author. Email: coppack@vogelwarte-helgoland.de ABSTRACT: Current climate change is affecting the timing of life-history events of birds, such as the timing of spring arrival at the breeding grounds. Interspecific differences in the advancement of spring migration have hitherto been attributed to differences in exposure to climatic variables in winter or on migration, without a more detailed consideration of sex-specific differences in the timing and extent of migration. Since males and females migrate during different times of the season or may segregate into different wintering grounds, we expect climate change to differentially affect the arrival timing of males and females. Furthermore, sexual selection theory predicts that the degree of protandry (i.e. the time difference between male and female arrival) should increase when the forces of natural selection opposing early arrival relax and the earliest males benefit from improved mating opportunities. Here, we explore whether the degree of protandry has changed during a period of climatic warming using long-term phenological data collected at 5 North European bird observatories. We show for 4 sexually dichromatic songbird species that spring migration has generally advanced, but that the degree of protandry has not changed. Moreover, we provide evidence that variation in large-scale weather conditions, as reflected in the North Atlantic Oscillation (NAO) index, have not affected the degree of protandry. We hypothesize that a parallel shift in the timing of male and female migration may be due to similar phenotypic plastic responses of males and females to climatic changes or reflect a correlated selection response in males and females. In addition, we discuss the possibility that the observed variation in protandry could result from seasonal shifts in local environmental conditions under which birds of both sexes ‘fall out’ and are sampled. KEY WORDS: Differential migration · Ficedula hypoleuca · Lanius collurio · Migration timing · Phenology · Phoenicurus phoenicurus · Protandry · Sylvia atricapilla Full text in pdf format PreviousNextCite this article as: Rainio K, Tøttrup AP, Lehikoinen E, Coppack T (2007) Effects of climate change on the degree of protandry in migratory songbirds. Clim Res 35:107-114. https://doi.org/10.3354/cr00717 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in CR Vol. 35, No. 1-2. Online publication date: December 31, 2007 Print ISSN: 0936-577X; Online ISSN: 1616-1572 Copyright © 2007 Inter-Research.

Tardy females, impatient males: protandry and divergent selection on arrival date in the two sexes of the barn swallow

Protandry reflects the earlier arrival of males than females to the site of reproduction. Such protandry is hypothesised to arise from sex differences in costs and benefits of early arrival. I investigated temporal patterns of arrival date of male and female barn swallows Hirundo rustica and temporal patterns of selection to test the hypothesis that sex differences in selection account for sex differences in arrival date. Mean arrival date of male barn swallows but not of females advanced during the last 33 years, giving rise to an increasing sex difference in arrival date. Early arrival was favoured by increasingly better survival in males, while females showed an opposite pattern that did not reach significance, although the effect differed between sexes. Early arrival increased fecundity in both sexes equally.The sex difference in viability selection in relation to arrival date changed from positive to negative as the degree of protandry increased in recent years, although there was no similar significant relationship for fecundity selection. Furthermore, sex differences in viability selection in a given year affected the degree of protandry in the following year through differential survival of certain phenotypes over others. Finally, temporal changes in sex difference in viability selection and protandry were related to an increase in the interval between first and second clutches, as the duration of the breeding season increased because of climatic amelioration. These findings suggest that arrival date is under divergent selection in the two sexes, providing a mechanism for the evolution of protandry.

Protandry, sexual selection and climate change

AbstractProtandry refers to the earlier appearance of males before females at sites of reproduction. Sexual selection has been hypothesized to give rise to sex differences in benefits and costs of early arrival, thereby selecting for earlier appearance by the sex subject to more intense sexual selection. If sexual selection is more intense, there is a greater premium on early arrival among individuals of the chosen sex because of direct selection for earlier arrival. This hypothesis leads to the prediction that changes in the costs and benefits of early arrival related to changes in environmental conditions should particularly affect the sex that arrives first and hence the degree of protandry. I tested this hypothesis using the Barn SwallowHirundo rustica. During 1971–2003, the degree of protandry increased significantly in a Danish population because males advanced arrival date while females did not. This earlier arrival by males compared with females was correlated with a significant increase by over 1.2 standard deviations in the length of the outermost tail feathers of males, a secondary sexual character, suggesting direct selection on both protandry and the secondary sexual character. Environmental conditions during spring migration in Northern Africa, as reflected by the normalized difference vegetation index, have deteriorated since 1984, resulting in increased mortality among males during spring migration, but not among females, and this deterioration of climatic conditions was positively correlated with an increasing degree of protandry. Likewise, an increase in April temperatures at the breeding grounds during recent decades is positively correlated with increased protandry, apparently because males can arrive earlier without increasing the fitness cost of early arrival. Local population size did not predict changes in arrival date. These findings suggest that rapid changes in climate can cause a change in degree of protandry and secondary sexual characters.

Protandry and Its Relationship with Adult Size in Some Spanish Stoneflies Species (Plecoptera)

Protandry in 11 stonefly (Plecoptera) species (representing five families) from Sierra Nevada (Spain) has been studied (recorded and quantified) over their flight period. All species exhibited a positive protandry coefficient (range, 0.07-0.33), indicating that no species shows proteroginy. Protandry was less pronounced in species having extended flight periods, brief flight periods, or highly concentrated flight maximums. The degree of protandry did not correlated with the degree of sexual dimorphism in size, contradicting the theoretical prediction of increased dimorphism with increased protandry.

Protandry models and their application to salmon

Mating systems characterized by restricted breeding seasons, male polygamy, and female monogamy are common among animals. In such systems (e.g., butterflies), the earlier emergence of males than females to breeding areas (protandry) is a typical phenological pattern. Protandry likely results from a timing strategy that maximizes mating opportunities by males. In Pacific salmon (Oncorhynchus spp.), males typically arrive at the spawning grounds in advance of females. Using arrival-timing models, I found that under the mate-opportunity hypothesis, the mating system of salmon favors protandry. Protandry is predicted under a range of competitive scenarios, and the degree of protandry is especially sensitive to the duration of male spawning activity. Greater protandry is expected with increasing population sex ratio (i.e., more males) when there is mate guarding, but lower protandry is expected with increasing population sex ratio when interference competition among males reduces male longevity. The timing of unequal competitors is expected to be similar, but among years, protandry may be less variable in the better competitor. Key words: arrival timing, Onchorhynchus, protandry, salmon. [Behav Ecol 13:337–343 (2002)]

Ejaculate production schedule and the degree of protandry in fishflies (Megaloptera: Corydalidae)

1. To examine the relationship between male–female emergence patterns and ejaculate dynamics, patterns producing sperm and seminal fluids in male internal reproductive organs, the size of a spermatophore transferred at mating and the fate of spermatophore contents moved into female storage organs were compared among the fishflies (Megaloptera: Corydalidae: Chauliodinae), Parachauliodes continentalis, P. japonicus and Neochauliodes sinensis. 2. Spermatophore contents moved into female storage organs decreased rapidly in P. japonicus and N. sinensis, but hardly at all in P. continentalis. This suggests that the females of the former two species may remate sooner than the latter species as it is known in insects that material remaining in the storage organs mechanically inhibits receptivity to mating. 3. Male P. japonicus and N. sinensis increased in internal reproductive organ mass continuously after adult eclosion, and the spermatophore size produced at the first mating increased with male age. In contrast, the internal reproductive organs of P. continentalis were relatively small and did not increase in mass after emergence. P. continentalis transferred a constantly smaller spermatophore at any copulation than the former two species. 4. Males of P. japonicus and N. sinensis emerged earlier than females, while P. continentalis showed a nearly simultaneous emergence pattern between the sexes. It seems that males of P. japonicus and N. sinensis (more polyandrous than P. continentalis) inhibit female receptivity for a longer time period by emerging earlier and transferring larger ejaculates. In these two species, the number of sperm ejaculated at the first mating also increased with male age. The increase in sperm number by emerging earlier may be adaptive for the males owing to numerical sperm competition when the female remates.

Small males emerge earlier than large males in Dawson's burrowing bee (Amegilla dawsoni) (Hymenoptera: Anthophorini)

Females of Dawson's burrowing bee (Amegilla dawsoni) are receptive to the males as they emerge but have become unreceptive by the time they begin to nest. In addition, there is a single emergence period per year lasting about a month. These factors are predicted to lead to protandry and males of Dawson's burrowing bee do tend to emerge earlier in the annual flight season than females. Moreover, even during a single day, emerging males tend to precede females. The degree of protandry, however, is size‐dependent, with smaller males tending to precede larger ones, both over the course of the flight season and on any given day. Because small males are at a disadvantage in the fights that occur for females, the earlier emergence of minor males may be a sexually selected response that reduces the likelihood that they will be displaced from potential mates by larger rivals.

Male emergence timing and mating success in the funnel‐web spider, Agelena limbata (Araneae: Agelenidae)

AbstractField observations on the relationship between male mating success and emergence timing in the funnel‐web spider, Agelena limbata, were conducted. Agelena limbata is an annual species and adult males appear slightly earlier than adult females in July. As males deposit a copulatory plug at the female epigynum after copulation, copulation with virgin females is important to males. The number of copulations in males with virgin females, which strongly correlates with the longevity of males and the number of females that males courted, did not correlate with the emergence timing of males. Early emerged males and females were significantly larger in size than later ones, but the correlation coefficient between the emerged date and the cephalothorax width was not strong. Males that emerged earlier did not have any advantage in copulating with larger and more fecund females. Furthermore, virgin females first copulated on average 7.9 days after their final molt and the mortality rate of adult males increased after the final molt. These factors may favor the smaller degree of protandry in male emergence timing in A. limbata.

The pollen‐collecting hairs of Campanula (Campanulaceae). II. Function and adaptive significance in relation to pollination

A peculiar and often discussed feature in the genus Campanula is the presence and function of the pollen‐collecting hairs (PCHs), covering the style and the dorsal surface of the stigmatic lobes on the pistil. The hairs sweep up the pollen as the anthers dehisce, and this “stylar brush” serves as a secondary pollen presentation mechanism, facilitating the transfer of pollen from the flower to visiting insects. The adaptive significance of the hairs in relation to pollination has been variously interpreted by several authors during the past two centuries, but a satisfactory explanation for the floral mechanism is still lacking. The aim of the present investigation is to elucidate the function of the PCHs by experimental manipulations of the male phase. Comparisons are made 1) among species with various degrees of protandry and different mating systems, and 2) between annuals and perennials. The results clearly show that the hairs serve not only as pollen‐collectors, but also as an important and efficient mechanism controlling stigma spreading and promoting cross‐pollination. A tactile stimulation of the hairs causes a shortening of the length of the male phase and accelerates stigma maturation. The more frequently the hairs are treated, the shorter the male phase. Similar trends were found in all species regardless of differences in the degree of protandry, life span, or mating system. Geographically well‐separated populations in two species showed differences in length of the male phase, depending on mating system and/or weather conditions. The relevance of the method used in this study compared to pollinator activity and its effect on the PCHs is discussed. The consequences of the retraction of the hairs with respect to self‐pollination are briefly discussed.

The Pollen-Collecting Hairs of Campanula (Campanulaceae). II. Function and Adaptive Significance in Relation to Pollination

A peculiar and often discussed feature in the genus Campanula is the presence and function of the pollen-collecting hairs (PCHs), covering the style and the dorsal surface of the stigmatic lobes on the pistil. The hairs sweep up the pollen as the anthers dehisce, and this “stylar brush” serves as a secondary pollen presentation mechanism, facilitating the transfer of pollen from the flower to visiting insects. The adaptive significance of the hairs in relation to pollination has been variously interpreted by several authors during the past two centuries, but a satisfactory explanation for the floral mechanism is still lacking. The aim of the present investigation is to elucidate the function of the PCHs by experimental manipulations of the male phase. Comparisons are made 1) among species with various degrees of protandry and different mating systems, and 2) between annuals and perennials. The results clearly show that the hairs serve not only as pollen-collectors, but also as an important and efficient mechanism controlling stigma spreading and promoting cross-pollination. A tactile stimulation of the hairs causes a shortening of the length of the male phase and accelerates stigma maturation. The more frequently the hairs are treated, the shorter the male phase. Similar trends were found in all species regardless of differences in the degree of protandry, life span, or mating system. Geographically well-separated populations in two species showed differences in length of the male phase, depending on mating system and/or weather conditions. The relevance of the method used in this study compared to pollinator activity and its effect on the PCHs is discussed. The consequences of the retraction of the hairs with respect to self-pollination are briefly discussed.

Absence of Trade‐Offs Between Sexual Size Dimorphism and Early Male Emergence in a Butterfly

Protandry, here defined as the earlier emergence of males, is a common feature in life histories and could be the result of sexual selection on males to maximize matings, or alternatively and incidental by—product of other selection pressures on the sexes. If protandry is selected for per se, theory predicts that it should be associated with seasonal environments where there is little overlap between generations. The degree of protandry should be insensitive to environmental conditions. Moreover, on the assumption that males and females grow at the same rate as larvae, a trade—off between development time and size is expected to result in a strong association between protandry and female—biased sexual size dimorphism. These predictions were tested by a combination of comparative and experimental studies on five populations of the speckled wood butterfly, Pararge aegeria, from central and south Sweden, England, Spain, and the island of Madeira. Protandry was associated with seasonal environments, as it was only exhibited in the three northernmost populations. Protandry in these populations remained largely constant in a variety of temperatures, both under direct development, when protandry results from a sex difference in development time through the egg, larval, and pupal stages, and under diapause development, when it results from a sex difference in pupal development time only. These results indicate that protandry is selected for per se through sexual selection in seasonal environments. Similar female—biased size dimorphism occurred in protandrous and non—protandrous populations alike, and hence sexual size dimorphism in P. aegeria is not a result of selection for protandry, nor the causal factor behind protandry. Protandry and sexual size dimorphism appear to be largely decoupled traits in the life history evolution of P. aegeria. This is achieved by means of variation in pupal developmental time and variation in the relative growth rates of the sexes. Variation in growth rates is likely to be a general phenomenon and may make possible independent optimization of size and development time (age at sexual maturity), and accordingly influence expected patterns of size—related trade—offs.

Morphological and growth rate differences among outcrossing and self‐pollinating races of Arenaria uniflora (Caryophyllaceae)

AbstractPopulations of Arenaria uniflora exhibit intraspecific variation in floral size and degree of protandry in association with the evolution of self‐pollination. Heterochrony, or a simple change in the absolute timing or rate of developmental events, is proposed as the evolutionary mechanism underlying the origin of the small, self‐pollinating flowers from their large, outcrossing progenitors. Inflorescence growth in two autogamous populations and their related outcrossing progenitors was studied to provide the temporal data necessary for testing the hypothesis of heterochrony. All four races showed significant variation in the growth and mature length of inflorescence organs. Inflorescences of selfing races were smaller, and had slower relative growth rates and a two‐fold increase in the plastochron relative to outcrossing populations. The large‐flowered races were both significantly protandrous. A more detailed growth analysis of flower development in two races indicated that the selfing flowers develop at a slower rate and for a longer duration relative to outcrossing flowers. The implications of these temporal changes in floral ontogeny for the heterochronic origin of self‐pollinating floral forms are considered.

Influence of Reproductive Ecology on Feasibility of Mass Trapping Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae)

(1) The degree of protandry, the average age at which females mated and the effects of size and adult nutrition on the reproductive performance of male western corn rootworms, Diabrotica virgifera virgifera, were evaluated to assess the potential of male removal as a management tactic for this pest. (2) Although some males emerged earlier, most males emerged at the same time as females. Dissections of teneral and young non-teneral females, as well as observations of newly emerged females in cornfields, showed that most females mate during the first 24 h after emergence. The synchrony between bimodal temporal distributions of mating and adult emergence, which reached peaks in the morning and the late afternoon/early evening, supported this conclusion. (3) When supplied an excess of virgin females in field cages, large males usually initiated mating at an earlier age and mated more often than small ones. However, neither longevity nor the mass of spermatophores were related to male size. Thus, small males, which sometimes mate twice daily, transfer a greater proportion of their resources at each mating. (4) Males which were provided young silks, ears of corn, leaves and tassles for the first week after emergence, after which they were only given leaves (i.e. poor food) died sooner and mated less often than similar sized males given continuous access to young silks, ears of corn, leaves and tassles (i.e. best food). The mass of spermatophores produced by males in the best food treatment was larger than those produced by males fed poor food. Spermatophore mass was positively correlated with age, especially during the first 10 days after emergence. (5) The reproductive potential of populations of D. virgifera virgifera males far exceeds the number of available virgin females in cornfields. Thus, intraspecific competition between males for mates should be considerable in this system. (6) The results of this study indicate that tactics relying exclusively on male removal do not have a large potential to manage D. virgifera virgifera populations.

Evolution of Self-Pollination in Granite Outcrop Species of Arenaria (Caryophyllaceae). III. Reproductive Effort and Pollen-Ovule Ratios

Patterns of resource allocation and pollen-ovule ratios were measured in nine pop- ulations of Arenaria uniflora, a winter annual endemic to rock outcrops in the southeastern United States. Dry weights of plants from cross-pollinating populations were greater than those from self- pollinating populations for both vegetative and reproductive biomass. Reproductive effort (pro- portion of total biomass represented by reproductive structures), however, was greater for selfers. Pollen-ovule ratios showed a close fit to expectations: low pollen-ovule ratios are associated with low rates of outcrossing (assessed in terms of degree of protandry). Number of pollen grains per flower and pollen-ovule ratios show no variation between years and little variation within and among flowers or plants. Number of ovules per ovary is less stable. It is suggested that use of pollen-ovule ratios obscures the evolutionary dynamics of forces influencing pollen grain number and ovule number. Pollen and ovule production should be studied as separate, and potentially

THE EVOLUTION OF SELF‐POLLINATION IN GRANITE OUTCROP SPECIES OF ARENARIA (CARYOPHYLLACEAE). IV. CORRELATED CHANGES IN THE GYNOECIUM

Intraspecific variation in gynoecial characters was analyzed by SEM for nine populations of Arenaria uniflora, a winter annual endemic to rock outcrops in the southeastern United States. Style lengths in self‐pollinating populations, formerly called A. alabamensis, were significantly shorter than in outcrossing populations, and style length showed a very strong correlation (r = 0.97) with outcrossing potential as assessed by degree of protandry. Selfers were also characterized by the production of fewer but longer stigmatic papillae. Papillae extended to the base of the style and diverged from all sides. Outcrossers had longer styles with dense but short papillae, mostly restricted to the upper surface of the style and lacking at its base. Styles also were observed to curl downward in outcrossers. These fine‐scale changes appear to enhance the effectiveness of plants as either outcross‐pollen or self‐pollen receivers.

The Evolution of Self-Pollination in Granite Outcrop Species of Arenaria (Caryophyllaceae). IV. Correlated Changes in the Gynoecium

Intraspecific variation in gynoecial characters was analyzed by SEM for nine populations of Arenaria uniflora, a winter annual endemic to rock outcrops in the southeastern United States. Style lengths in self-pollinating populations, formerly called A. alabamensis, were significantly shorter than in outcrossing populations, and style length showed a very strong correlation (r = 0.97) with outcrossing potential as assessed by degree of protandry. Selfers were also characterized by the production of fewer but longer stigmatic papillae. Papillae extended to the base of the style and diverged from all sides. Outcrossers had longer styles with dense but short papillae, mostly restricted to the upper surface of the style and lacking at its base. Styles also were observed to curl downward in outcrossers. These fine-scale changes appear to enhance the effectiveness of plants as either outcross-pollen or self-pollen receivers.

Endokrinologische und genetische Untersuchungen an einer proterandrischen Population des Polychaeten Ophryotrocha labronica

Decerebrate, sexually immature juveniles of Ophryotrocha labronica develop into males and females, although the normal ratio observed in intact worms is not attained. Even without the prostomium, primary males split into secondary females and stable males. Stable males and females do not show any reaction after decerebration as far as sexual differentiation is concerned. The females proliferate new sets of oocytes and the males continue spermatogenesis. In a great number of cases, decerebrate secondary females resume spermatogenesis. Amputation of the prostomium induces the development of the k-jaw. Even in small individuals below the size at which the k-jaw normally appears, the change to the k-jaw is induced. Heterospecific transplantations of prostomia from female O. labronica donors to decerebrate O. puerilis hosts revealed that the prostomia of female O. labronica produce and release a substance which completely compensates the lack of the host's own prostomium. Obviously, this ootropic substance is not necessary in all cases for gaining and/or maintaining female differentiation in O. labronica. Although it is produced both in stable females and secondary females, only a portion of the secondary females requires this substance for female differentiation. It is, therefore, presumed that the strength or stability of female differentiation expresses itself by hormone-dependent or independent oogenesis. In crosses with a gonochoristic population of O. labronica, protandric hermaphroditism is restricted to a few descendants. Crossing experiments within the protandric population of O. labronica seem to indicate a genetic component in the degree of protandry and in sex determination.

Germ Cell Migrations in a Cestode

USUALLY in cestodes the potential oocytes and associated cells of the female system leave the germinal primordium in the median line of the proglottid while the cells are still either undifferentiated or not easily distinguishable. In a member of the problematical order Tetrabothridea, oogonia, true vitelline cells, and what are considered to be shell-producing cells of two kinds, differentiate within the female primordium, although the whole process is delayed in this worm because of marked protandry. This delay causes the process of germ cell differentiation to coincide in time with the differentiation of the uterus, and the latter can thus be identified, for the first time, as the outer layer of the primordium enclosing the germ cells except for a posterior aperture (Fig. 1). The shell-producing cells and oogonia migrate through this aperture in an organized sequence: first the compact gland cells, usually called ‘vitelline’, then the cells of a diffuse gland similar to those of the follicular glands of Pseudophyllidea, also usually called ‘vitelline’, and finally the oogonia. The true vitelline cells, arranged in the primordium in rosettes around the oogonia, degenerate within the primordium. The three sets of migrating cells, some of which were seen to be amoeboid, travel progressively less far into the medulla, which is itself differentiating at the same time (Fig. 1). The distance covered by the migrating cells would appear to be related to this medullary differentiation and thus indirectly to the degree of protandry. The migrating cells form two organs and not three as the diffuse gland contains both shell-producing cells and oogonia (Fig. 1).