Cytological observations on fertilization in the carp, <i>Cyprinus</i> <i>carpio</i> L.

Abstract

Employing the carp, Cyprinus carpio L. as the meterial, the processes of maturation of the egg and the phenomena of fertilization were microscopically observed in this study. Observations were carried on in the sections of the eggs, fixed at different intervals ranging from five minutes to 2 hours after fertilization. The majority of eggs were fixed in Carothers' solution and some were preserved in Gilson's fluid. For sectioning the Gilson material proved to be better than the Carothers material. Sections were made following the usual paraffin method. They were stained with Ehrlich's haematoxylin. The results of observations are briefly outlined as follows:1) The first maturation division of the egg is completed prior to ovulation within the ovary. After the extrusion of the first polar body, the chromosomes remaining in the egg are immdiately drawn into the equatorial plate forming the metaphase spindle of the second maturation division (Fig. 4).2) The spawning and fertilization take place in the water at the same time.3) At the time of insemination the second maturation division of the egg persists in the stage of metaphase, and then it gradually advances in further course of division (Fig. 8). About 25 to 30 minutes after insemination the second division is completed, resulting in the formation of the second polar body (Fig. 9).4) During the ovulation process the course of the second division does not proceed being arrested in metaphase. Stages subsequent to the metaphase of the second division take place only after the entry of the spermatozoon into the egg. This fact emphasizes that insemination is a necessary antecedent to the second maturation division to advance further from metaphase.5) Only a single spermatozoon enters an egg (Fig. 7). Thus the carp egg is monospermic in fertilization under the normal condition.6) About 40 to 50 minutes after insemination there is found at the animal pole of the egg the already metamorphosed female pronucleus. It assumes a spherical shape lying in naked state in the ooplasm (Fig. 13).7) As early as 50-60 minutes after entrance, the head of the spermatozoon is converted into the male pronucleus attaining nearly spherical form (Figs. 10-12). At this stage there is no visible demarkation between the male and female pronuclei, so far as their structure and size are concerned. But there is present a ramarkable character peculiar to the male pronucleus. That is the pronounced appearance of the sperm-aster.8) After completion both the pronuclei begin to move towards the deeper part of the animal pole of the egg to conjugate with each other (Fig. 14). The conjugation of the pronuclei generally takes place about 1 hour after insemination occurs (Figs. 15-17).9) After the two pronuclei conjugate together, they do not actually fuse, but lie side by side in closs contact with the nuclear membrance intact (Figs. 15-16). Thus the maternal and paternal chromosome materials are separated in distinct groups during the stages prior to the first cleavage division. The chromosomes are formd independently in each vesicle (Fig. 17).10) In the egg about 11/2 hours after fertilization, the first cleavage spindle is generally formed (Fig. 18).