Gonadal Type Research Articles

The gonadal anatomy and sexual pattern of the protandrous sex‐reversing fish, Rhabdosargus sarba (Teleostei: Sparidae)

The gonadal anatomy and sexual pattern of Rhabdosargus sarba (Teleostei: Sparidae) was studied to provide some basic structural information for the subsequent investigations on the endocrinology of protandrous hermaphroditism in this fish. Evidence derived from relating sex to differences in body size, from gonadal histology and from biopsy, revealed the occurrence of natural sex reversal from male to female in this species. The gonad of R. sarba possessed topographically distinct male and female zones that were well separated by connective tissue. Based on gross‐anatomical and histological observations, four types of gonad were distinguished and were designated as Types I‐IV in the present study. Active spermatogenetic tissue was present in the Type I, II and IV gonads. Dormant oogonia and perinucleolar oocytes were found in the Type I (male) and II (intersex) gonads, respectively. In the Type III (female) gonad, a functional ovarian zone was observed and the testicular tissue was vestigial. The existence of ovarian tissue as an oogonial band in the Type IV (male) gonad, which was found more commonly in large specimens, suggested that these functional males might not undergo sex reversal in their life cycle. The interrelationship of the different types of gonad is discussed with reference to protandrous hermaphroditism.

The importance of surgical evaluation of patients with “Turner-like” sex chromosomal abnormalities

Children with "pure" gonadal dysgenesis do not require abdominal explorations to remove their gonads, because the risk of development of malignancy is minimal. Children with "mixed" gonadal dysgenesis, who usually present with sexual ambiguity, need to have gonadectomy because of the high incidence of malignancy in these gonads. We present four children (three adolescents and one two year old) who presented with minimal somatic anomalies, who did not have sexual ambiguity. The prominent features were primary amenorrhea and short stature in the adolescents, and minor dysmorphic features in the small child. On the basis of these physical features, complete genetic evaluation was carried out, and a Y chromosome or an unidentifiable fragment was identified in all four cases. At exploration, bilateral genital streaks were found, and were removed. In addition to the usual microscopic features of the streaks, Leydig cells and tubular structures resembling rete testis were seen in all the gonads. In one gonad, a gonadoblastoma was found. Since this type of gonad is "at risk" for the development of gonadoblastoma, the need for complete genetic evaluation of all females with "Turner-like" chromosomal abnormalities, although phenotypically female, is emphasized. If a Y chromosome or any unusual fragment is identified, exploration should be carried out, and any gonad that is not a normal ovary should be removed.

A Rapid Method of Quantitating Steroids Resulting from the Incubation of Gonadal Tissues with Radioactive Precursors

Abstract A rapid method has been developed for the quantitation of steroid metabolites resulting from the incubation of specific gonadal cell types or gonadal tissue with radioactive precursors. The method involves the use of high performance liquid chromatography (HPLC) for separating the steroids and a flow-through radioactive detector (Flo-One HP) for quantitating the radioactive 3H precursor and metabolites in the presence or absence of 14C-steroid recovery tracers. A comparison is made between the results obtained directly by the Flo-One HP radioactivity detector and the fraction collection method, (counting aliquots from individual fractions in the liquid scintillation counter). In addition, the results using an electronic stream splitter in the analysis of a percentage of the effluent directly by Flo-One HP are evaluated. The remaining percentage is collected in a fraction collector and is used for further analysis (e.g. recrystallization, RIA, further purification and characterization).

H-Y antigen studies in thirty patients with abnormal gonadal differentiation: correlations among sex chromosome complement, H-Y antigen, and gonadal type.

The expression of H-Y antigen was examined in 30 patients with abnormal gonadal differentiation. Four patients with XX true hermaphroditism and 8 with XX male syndrome were all H-Y positive, in spite of the absence of cells bearing Y chromosome in the body. Of the 7 patients studied with XY gonadal dysgenesis, 6 paitents were H-Y positive and one was negative. The reduced antigen titer as compared to normal males was observed in one of the H-Y positive patients. Clinical and cytogenetic studies in the total 30 patients adding 4 of mixed gonadal dysgenesis and 3 of male pseudohermaphroditism, could not establish strict correlations among sex chromosome complement, H-Y antigen, and gonadal type of patients. However, a distinct association was recognized; when testicular structure was seen in the gonad, the H-Y antigen of the patient was always positive regardless of sex chromosome complement. In this study, our modified method and technical problems on the H-Y antigen examination are described, and genetic mechanisms for the regulation of H-Y antigen expression are discussed.

The morphology of fetal gonads of spontaneous mouse hermaphrodites

The gonads of 107 spontaneous, chromosomal mosaic, day-15 fetal hermaphrodites derived from BALB/cWt strain male mice are described and photographs of seven gonads representative of the major types are presented. There were 16 ovaries, 15 testes, and 183 ovotestes. The ovotestes contained on the average more testicular tissue than ovarian, and the ovarian tissue was more frequently located at the gonad poles, particularly the craniad pole. There was no difference between left and right sides with regard to gonad type, but more pure gonads were found on the left than on the right side (21/10), Meioses were observed throughout the ovarian tissue and also in some testicular cords, particularly in the caudad pole of the gonad. Some meiotic figures contained sex vesicles. The significance of these findings is discussed in relation to several aspects of sex determination.

Chromatin diminution and a chromosomal mechanism of sexual differentiation in Strongyloides papillosus.

Eggs obtained from feces of rabbits infected with Strongyloides papillosus were squashed and the karyotypes were determined. They contained cells with either two long and two medium sized chromosomes (2L2M), or one long, three medium and one short chromosome (L3MS). Two types of parasitic female gonad could be distinguished on the basis of oocyte chromosome morphology at prometaphase of the maturation division. All the oocytes in a gonad contained either two upaired long chromosomes and two unpaired medium sized chromosomes, or two unpaired medium sized chromosomes and two unpaired chromosomes segmented into beads in one region. At the maturation division in mitotic parthenogenesis the beads appear to be lost from one of the chromosomes. This generates a medium sized and a shorter chromosome, which together with the undiminished chromosomes make up the L3MS karyotype. Animals with beaded oocyte chromosomes lay eggs that develop into males. It is suggested that males are heteromorphic for the long homologue due to chromatin diminution, that occurs in the maturation division of mitotic parthenogenesis.

Growth and sexual differentiation in the gonads of chick and duck embryos

ABSTRACT The sex-dependent morphological asymmetry between gonads of bird embryos is generally considered to represent the critical point at which the definitive pattern of sexual differentiation is established. A quantitative study of growth in gonads of chick embryos (6- to 11-day-old) and of duck embryos (7- to 11-day-old) has been carried out in order to clarify in what manner growth of gonads is involved in sexual differentiation. Growth has been estimated by the determination of total protein and DNA content, and of DNA synthesis [3H]Tdr incorporation in vitro). Increase of protein and DNA content follow different growth curves according to the sex and the laterality of gonads. As early as day 6 (chick) or day 7 (duck) significant differences of protein content exist between gonads: left > right in both sex embryos, and male > female on both sides. Differences in DNA content of gonads are large even at the earliest stages. In the 6-day-old chick embryo both testes contain less DNA than the left ovary and more DNA than the right ovary. In the 7-day-old duck embryo both testes contain the same amount of DNA as the left ovary and twice as much DNA as the right ovary. Consequently, the protein/ DNA ratio has a different value according to the sex of the gonad. In the 6-day-old chick embryo and in the 7-day-old duck embryo absolute values of DNA synthesis (cpm/gonad) are higher in left gonads than in right gonads, and higher in male than in female gonads. When calculated as a ratio to the protein content (cpm/protein) DNA synthesis is lower in the left ovary than in the three other types of gonads, both in chick and duck embryos. When calculated as a ratio to the DNA content of gonads (cpm/DNA), DNA synthesis is lower in the left ovary than in the three other gonads in the chick, and lower in the left ovary and in testes than in the right ovary in the duck. The results show that growth of gonads is sex-dependent at a very early stage. The meaning of this sex character is discussed with special reference to the role of steroid hormone secretion.

Chromosomes and the gynecologist

Recent advances in cytogenetic techniques made a valuable contribution toward the modern practice of obstetrics and gynecology. The state of the art regarding the application of these techniques is reviewed in the following areas: the clinical features related to the various sex and autosomal chromosomal anomalies, the cytogenetics of gynecologic malignancies, the chromosomal analyses of spontaneous abortion and of parents with habitual abortions. Chromosome studies in male infertility revealed abnormalities in 11.5 per cent of 69 patients with azoospermia and 9.1 per cent of 165 patients with oligospermia. Among 77 patients with primary amenorrhea, 27.3 per cent revealed chromosomal abnormalities compared to 3.8 per cent in 103 patients with secondary amenorrhea. The term “ovotesticular dysgenesis” is used for the first time in the literature to describe a specific histologic type of streak gonad which contains ovarian stroma and dysgenetic testicular tubules.

Sex differentiation in a cichlid, Tilapia zillii.

This report deals with the development of gonadal sex differentiation in a cichlid, Tilapia zillii. At the time of hatching, primordial gonad was not yet formed, but primordial germ cells were found to be located between the mesonephric duct and the gut in the posterior trunk region. At the stage of 15 days after hatching, two types of gonads were distinguishable. One contained many germ cells, some of which showed meiotic prophase. The other contained a few germ cells without any in meiotic prophase. During the course of further development, the former differentiated into ovary, the latter into testis, respectively. The so-called testis-ova were detected in the developing testes, which were first detectable at the stage of 40 days. Concomitantly with the proceeding of spermatogenesis, they increased in number and size. The testis-ova showed irregular shape and their cytoplasm was stained deeply with hematoxylin. The largest testis-ovum was 60μm in diameter, which was much smaller than the oocytes in the ovary.

Surgical Construction of the Male External Genitalia

In male pseudohermaphrodites and true hermaphrodites, selection of the sex of rearing should be made according to the anatomy of the external genitalia, particularly the size of the phallus or the presence of a serviceable vagina, without regard to the type of gonad, the chromosomal pattern, or the structure of the internal genital tract.

Morphological Differences in Male Gonads Among Nine Genera of Gadidae (Pisces)

Among 12 species of Gadidae, which belong to nine genera and three subfamilies six basic types of male gonads are observed. In the freshwater genus, Lota, gonads are simple nearly cylindrical m shape, and not subdivided into lobes. In the marine genera of cold waters, such as Melanogrammus and Gadus, the other extreme in gonad structure, consisting of numerous, rather uniform convolutions, is found. In the marine genera Urophycis and Merluccius of more temperate waters, the male gonads consist of several lobes, rather irregular in shape and size. Among genera inhabiting brackish and sometimes fresh water, such as Microgadus, Boreogadus, and Eleginus, the gonads are subdivided into lobes which are not very numerous. In the genus Arctogadus, distributed in salt and brackish Arctic waters, the gonads are intermediate m their complexity between the typical marine genera of cold water, Melanogrammus and Gadus, and those three genera inhabiting brackish waters. The explanation for the morphological differences in male gonads of Gadidae appears to be associated principally with the respective type of waters (salt or fresh) which they inhabit and their spawning requirements and to a much lesser degree with their systematic relationships.

The mutagenic effect of N-nitrosomethylurea on the dumpy locus in Drosophila melanogaster

N-Nitrosomethylurea was used to induce mutational events at the dumpy locus of Drosophila melanogaster. The results show that a 0.1% concentration of the agent induces a mutation frequency of 1.8%, which is comparable to that found for ICR-170 or ethyl methanesulfonate. The mutant spectrum of F 1 dumpy mutants induced by N-nitrosomethylurea does not differ appreciably from the spectrum obtained following the application of ICR-170. It does differ from the spectrum of mutants for X-ray mutagenesis, which induces a class of variegated dumpy mutants not found for ICR-170 or N-nitrosomethylurea. The percentage of mutant somatic tissue within each induced dumpy mutant was calculated by the identification and isolation of mosaic (fractional) mutants and complete (whole body) mutants. Like ICR-170 mutagenesis, phenotypically mosaic mutants predominate in the progency counts produced by postmeiotic sperm. In contrast, phenotypically complete mutants predominate in progeny counts produced by postmeiotic sperm following X-ray mutagenesis. Approx. 25% of the fertile F 1 dumpy mutants transmitted a dumpy phenotype to the F 2 generation. This is similar to the 25% transmittance obtained for ICR-170 but it is considerably less than the 72% transmittance established for X-ray induced mutants. In addition, the transmission test showed that the fertile F 1 mutants, irrespective of their somatic mutant expression, may possess any one of three types of gonads; completely normal, completely mutant, or fractionally mutant. A translocation test, employed to detect chromosomal breakage showed a 0.5% frequency of translocations between the second and the third chromosomes. This low frequency of breakage, in conjunction with the absence of the variegated dumpy class, suggests that the mode of action of this agent is based on a molecular alteration of the genetic material within the dumpy locus rather than on breakage events involving the addition, rearrangement, or deletion of large pieces of the chromosome bearing the dumpy locus. N-Nitrosomethylurea induces a high incidence of sterility in the premeiotic progeny broods. The ability of N-nitrosomethylurea to react chemically with the DNA polymer by means of the diazonium radical and the effect that this would have on meiosis is proposed as the major mechanism leading to male sterility. The induced sterility cannot be attributed to the small amount of breakage induced by the mutagen.

Développement d’individus haploïdes adultes élevés en parabiose chez le triton Pleurodeles waltlii Michah.: Syndrome de l’haploïdie et différenciation sexuelle

ABSTRACT The development of adult haploid individuals of the urodele Pleurodele waltlii joined in parabiosis: the haploid syndrome and sexual differentiation Haploid individuals of the Urodele amphibian Pleurodeles waltlii, joined in parabiosis with diploid embryos of the same species at the tail-bud stage, were raised to adults. This is the first such case among vertebrates. The haploid individuals lived for 2 or 3 years and more. When they were sacrificed, nothing indicated a limit to their life-span. The hormonal secretions of the gonads are active and the target territories competent, as shown by the reactions of the Müllerian ducts, cloaca and callosities. The lethal haploid condition is overcome through cross-circulation by which the haploid benefits from the diploid’s blood. The diploid parabiont is not affected in its general structure by the presence of the haploid’s blood. Crosscirculation is demonstrated by the presence of both haploid and diploid erythrocytes in blood smears. The manifestations of the haploid syndrome obvious at the adult stage are described. The growth of both animals in the pair is greatly retarded; larval life is lengthened. This is not specific to the dual genetic constitutions, but results from the physiological conditions set up through parabiosis. Despite the restricted growth of the diploid partners, their gonads can undergo sexual maturation, i.e. produce spermatozoa and mature ovocytes. The haploid individuals, obtained by heat shock of the egg, were androgenetic. In the best cases, they differentiate testes having the general structure of an adult gonad. In such cases spermatogenesis begins, but the process fails at the stage of primary spermatocytes, during the first division. Bivalents do not form. The meiotic cycle can proceed up to anaphase of the first division. Afterwards, pycnosis is general in all testicular cysts. A poorly developed glandular tissue is built up in the cysts emptied of germinal elements. Usually, however, testicular development in the haploids is limited. The gonads are small and remain juvenile, a condition normally observed in young animals after metamorphosis. Sometimes the testes are even vestigial and sterile. These different conditions are best analysed in the pairs (♂ 2n/♂ n) in which antagonistic hormonal effects do not occur. It seems likely that the hypodevelopment of the haploid gonads is due, at least partly, to the physiological conditions of parabiosis and to endocrine deficiencies associated with the haploid syndrome. In heterosexual parabiosis a new factor intervenes, i.e. the antagonistic hormonal secretions of the gonads themselves, during and after sexual differentiation. Different reaction patterns displayed in parabiosis (♀ 2n/♂ n) are described: (a) Dominant inhibitory action of the haploid testes on the diploid ovaries. The effect is in accordance with the general case observed in pairs (♀ 2n/♂ 2n). (b) Simultaneous development of the two types of gonads. The ovaries reach the adult stage and activate the Müllerian ducts. The testes can reach a submature state where spermatogenesis begins. In such combinations, the Müllerian anlage of the male partner react to the feminizing effects of the ovaries, (c) Gynogenic dominant effect of the ovaries on the sexual differentiation of the male gonad. The latter is feminized into an ovotestis. In this case, haploid ovocytes with lampbrush chromosomes differentiate. The diploid partner’s ovaries remain immature through the androgenic effect of the testis, though their ovarian nature is not modified. This reaction corresponds to the one already described in heterosexual hetero specific, homoploid (♀ 2n/♂2n) combinations obtained between species in Triturus and Ambystoma.

Sex differentiation and gonadogenesis in lampreys (Part I and II)

The development of the gonad and the processes of sex differentiation have been studied in the ammocoete of Lampetra planeri, with particular emphasis on the mode of differentiation of the male gonads. After an initial undifferentiated stage during the first larval year, meiotic changes appear, particularly in cell nests, and from these germ cells and from isolated gonocytes, growing oocytes are developed. The initial phase of differentiation is regarded as a female stage, during which auxocytosis occurs on a variable scale in probably all gonads. This is followed in definitive males, by the degeneration of germ cells that have already differentiated in a female sense, either at the early meiotic prophase, or later in the cytoplasmic growth period of the occyte. Even in morphologically differentiated testes, meiotic activity and oocyte growth may be observed, although on a diminishing scale, throughout the greater part of the larval period. More extensive and synchronous atresia of growing oocytes may occur comparatively late in the larval period, giving the appearance of sex inversion. These cases are regarded as examples of abnormally retarded differentiation. Other presumptive male gonads are derived from gonads consisting predominantly of meiotic cysts, which have persisted beyond the age when auxocytosis is normally completed in definitive females. In some cases the future male gonad can be recognized at an early stage by certain morphological and histological criteria, irrespective of the condition of the germ cells. The views of Okkelberg (1921) and D'Ancona (1943, 1949) on the supposed existence of intersexual gonads have been discussed in relation to the quantitative analysis of gonad types. The author believes that the sexual indeterminacy attributed to these animals is illusory and that once differentiation has begun, its course and direction are irreversible. In male differentiation there is some evidence of renewed activity in the peritoneal epithelium of the gonad surface and it is suggested that the somatic elements of the testis may have some inductive effect tending to inhibit the further differentiation of bipotential germ cells in a female direction. No continuity exists between the undifferentiated cell nests of early stages and the cysts of germ cells in the mature testis. The former should not, therefore, be regarded as male elements in an hermaphrodite gonad. At early stages of differentiation, potential male ammocoetes are those in which the gonads are composed predominantly of meiotic cysts, or in which regression is taking place at various phases of oogenesis. The critical period for oocyte growth and for male differentiation occurs at lengths of 60 to 70 mm when the ammocoetes are at the beginning of their third year. From 50 mm upwards the proportion of female gonads consistently exceeds 50% of the total of all types. Analysis of ammocoete populations suggests that there is normally a slight excess of females, but that the sex ratios do not vary widely from one stream to another. No evidence has been found to suggest that environmental factors play any part in sex differentiation or that the sex orientation of the gonad is labile and indeterminate.

A study of the types of male gonads found in the Acrididae (Orthoptera)

A study of the types of male gonads found in the Acrididae (Orthoptera)

DEVELOPMENT OF THE PRIMARY GONADS AND DIFFERENTIATION OF SEXUALITY IN TEREDO NAVALIS AND OTHER PELECYPOD MOLLUSKS

Two more or less distinct types of primary gonads are found in bivalve mollusks. The simplest type occurs in several families of the order Prionodesmacea, where the profusely branching follicles on each side of the body are composed mainly of gametogenic cells and each follicle is nourished by the surrounding mesenchyme or vesicular connective tissue.In the second type, found mainly in the order Teleodesmacea, the branching follicles of the primary gonads are composed principally of large, vacuolated follicle cells of a nutritive nature, the primary gonia being scattered along the central axis or on the periphery. These nutritive cells are cytolyzed during gametogenesis.In some bivalves intermediate conditions are found, with associated gametogenetic follicles and nutritive tissues.In the ambisexual or hermaphroditic species, as Teredo navalis, the primary gonia are early differentiated into the two sexual types. Of these the spermatogonia usually, but not invariably, proliferate and complete gametogenesis in advance of the ovogonia, giving each follicle the appearance of a spermary with a layer of ovocytes on the periphery. As a general rule the ovocytes do not become fully mature until after the discharge of the spermatozoa. There are many variations in the relative proportion and time of spawning of the two types of gametes, however, and in exceptional cases the gonad is almost exclusively of the male or of the female type. Most individuals function first in the male phase, then change to the female phase, while some individuals experience in addition a second sequence of male and female phases. Less frequently a primary female phase precedes the first male phase. Individual differences in the conbinations of the modifying hereditary sex factors are presumably responsible for most of these variations.Each species studied, except those that are strictly unisexual, shows some variations in the sequence of male and female or functionally hermaphroditic phases.

THE EFFECTS OF ANDROGENS UPON THE FISH (LEBISTES RETICULATUS)*

THE RÔLE of sex hormones in early sexual differentiation, including the development of the gonad, is being intensively studied in many forms, including fish.1,2,3 The guppy, Lebistes reticulatus, is a convenient fish in which to make such studies. Berkowitz1 found that estrogens suppressed male and induced female secondary characters in male guppies if treatment began before sexual maturity; withdrawal of hormone treatment then permitted male characters to appear coexistent with induced female characters. He found, further, that estrogens had little if any effect upon the ovary, but in a high percentage of cases induced either a suppressed or hermaphroditic type of male gonad when given to males. This study is complementary to that of Berkowitz and is concerned with the effects of the androgen, testosterone propionate,‡ upon the above-mentioned phenomena. The results indicate, as in birds, that different types of control influence different secondary sexual characteristics and that the whole subject is one of great complexity.

Potencies of the gonad-forming area in the chick as tested in chorio-allantoic grafts.

1. This report deals with an analysis by chorio-allantoic grafting of the developmental potentialities of the gonad-forming areas of stages prior to the genital ridge in the chick embryo. The stages tested include a) those prior to the formation of the germinal epithelium (donors having from 29 to 34 somites) and b) those in which a germinal epithelium is visibly differentiated (donors having from 35 to 41 somites). In all of these stages germ-cells are present in the gonadforming areas. 2. a) In 34% of the 77 grafts histologically examined a gonad of specific sex has differentiated. Prior to the origin of the germinal epithelium such a gonad arises in 23.9% of the grafts whereas after its formation in 48.4% of the grafts; according to a former report (WILLIER '27) when the genital ridge stage is reached it forms in approximately 100% of the grafts. b) A gonad-like body of undetermined sex forms commonly (60%) in the grafts of the gonad-forming area whereas it rarely does so in grafts of the genital ridge. Prior to the 31-somite stage this type of gonad only has thus far formed. Several grades of organization of these bodies are recognized. c) The gonad-forming area yields somewhat frequently multiple gonads whereas a well-defined genital ridge gives as a rule a single gonad. d) The size of the gonad of specific sex which arises from the gonad-forming area is smaller than one from the genital ridge. It is thus evident that a progressive change in the developmental potentialities of the gonad-forming area occurs as it transforms into a genital ridge. This is interpreted as indicating that the gonad-forming area during its initial development exhibits an ascending organization which finally attains at the genital ridge stage a fully "determined" condition. 3. In the majority of the grafts of the gonad-forming area masses of germinal-cells are found external to the gonad in the mesenchyme or spaces within it. In such positions the germ-cells merely undergo repeated multiplication, exhibiting no tendency to form a gonad. If by chance germ-cells come to be situated in the wall of either the Wolffian or Müllerian duct, the overlying coelomic epithelium thickens into a layer simulating a germinal epithelium, yet the essential components of an avian gonad, namely, the sexual cords, fail completely to invaginate. The origin and differentiation of the germinal epithelium, on the contrary, is apparently independent of primordial germ-cells since a sterile gonad forms in grafts of whole blastoderms of early somite stages following the removal of the "germ-cell crescent" ofSWIFT. 4. Germ-cells in the mesenchyme remain cytologically undifferentiated, like the primordial germ-cells, whereas those which come to be situated in the sexual cords undergo specific differentiation. The conclusion is reached, therefore, that the germ-cell is dependent upon the specific tissue environment of the sexual cord for its sexual differentiation.

SEXUAL PHASES IN THE AMERICAN OYSTER (OSTREA VIRGINICA)

1. Examination of the developing gonads of young oysters from various localities at frequent intervals during the first two years of life shows that a primary bisexual gonad is formed in each individual within a few months after setting.2. The activities of the gonad depend upon the temperature of the water and apparently other conditions of nutrition, a much larger proportion of the animals becoming sexually mature during the first year in warmer than in cooler localities.3. The primary gonad contains the antecedent cells of both sexes, with ovocytes upon the walls of the follicles and spermatocytes intermingled and bordering the lumens.4. The protandric nature of the primary gonad frequently becomes manifest by the rapid proliferation of the spermatogonia and the formation of primary spermatocytes; the latter soon pass through the synaptic phases and lead to the production of secondary spermatocytes and spermatids at the age of a few months. But no functional spermatozoa have been observed until the following spring in the areas investigated. The species is not strictly protandric, however, for 3 to 30 per cent of the sexually mature yearlings are females, the ovaries of which have developed directly from the primary gonads without the completion of a preliminary functional male phase.5. The definitive sexual gland is a transformation of the primary gonad by the proliferation of spermatogonia and the disintegration of many of the ovocytes to form the spermary or, less frequently, the growth of ovocytes, accompanied by the disintegration of spermatocytes and such spermatids as may be present, to form an ovary. But the intersexual character is usually retained to at least some extent in both types of gonads.6. The proportion of male and female cells in the mature gonad is highly variable, a few large ovocytes being frequently found in some parts of otherwise typical spermaries, while in the ovary some follicles may retain characteristic male cells. True hermaphroditism was found in 1 to 4 per cent of the sexually mature oysters at the end of their first year. Apparently normal development follows selffertilization.7. In the warmer of the two principal localities investigated about 70 to 80 per cent of the oysters which became sexually mature during their first year were males. In the cooler locality the proportion of males exceeded 95 per cent.8. Most of the relatively small number of females are among the largest of their age group, indicating a close correlation between sex and size. This may imply that the female is metabolically the more active sex or that she requires better nutritive conditions in order to mature, or that sex in this species is so labile that the nutritive conditions of the individual at the critical period of sex differentiation determine which of the alternative types of cells in the primary bisexual gonad shall predominate. Alternative genetical explanations are discussed.9. After the animal has spawned as male or female the gonad may still retain its bisexual character; the sexual phase during the following year may again depend largely upon nutritive conditions.10. The primary bisexuality of this species and the cellular mechanism for sex reversal here reported are interpreted with reference to related species of the genus in which hermaphroditism and alternating sexual phases have been retained.