extend inward and fill up the lumen of the follicles. 

 Early at this stage the sex cells become dense and 

 opaque, a condition which interferes with cyto- 

 logical study. 



Gametogenesis of C. virginica and 0. lurida has 

 been studied by Coe (1932a, 1932b, 1934, 1936, 

 1938) and that of the Australian rock oyster, 

 C. commercialw, by Cleland (1947). Only the 

 main points of this process were disclosed by these 

 investigations. 



OVOGENESIS 



In all animals the primordial germ cells become 

 distinguishable as primary ovogonia in the females 

 or spermatogonia in the males after a certain num- 

 ber of divisions. After a period of quiescence they 

 begin to divide again and give rise to secondary 

 ovogonia or spermatogonia. After several genera- 

 tions the cells stop dividing and enter a growth 

 period, which is more prolonged in the females than 

 in the males. The growth period is characterized 

 by a series of cytological changes, each differing 

 from the preceding stage. The cells which \vill pro- 

 duce gametes are at this stage called auxocytes, 

 from the Greek "auxesis" meaning growth, and are 

 referred to as ovocytes in the female and sperma- 

 tocytes in the male. 



Ovogenesis in oysters begins with the appearance 

 of enlarged cells in the germinal epithelium. These 

 are the ovogonia, which in C. virginica and 0. 

 lurida are distinguished from other cells of the 

 germinal epithelium by their relatively large nuclei 

 with conspicuous nucleoli and loose chromatin 

 network (fig. 292, og). The ovogonia usually lie 

 next to tlie follicle wall, and their distal sides do 

 not protrude into the lumen. Differences between 

 the early ovogonia and indifferent residual cells (I) 

 are not conspicuous. Examination of a series of 

 sections and study of the sequence of changes in the 

 appearance and structure of the cells are necessary 

 to assure a positive identification. 



After one or two divisions the ovogonia change 

 in appearance as well as in size. This generation of 

 female sex cells called ovocytes can be recognized 

 by the presence of fibrillar mitochondrial bodies 

 (sometimes called yolk nuclei) , and by tlie spiremes 

 of densely packed chromosomes (figs. 293 and 294). 

 Their nucleoli become very conspicuous. 



Dm-ing the last stage of ovogenesis the ovocyte 

 begins to grow rapidly, and the distal part, gi'ossly 

 enlarged and rounded, protrudes into the lumen 

 of a follicle. At the same time the connection 



0C5 



Figure 292, — Follicle wall of the ovary of C. virginica. 

 I — indifferent residual cell; oc — residual ovocyte; ocs — 

 two young ovocytes in synaptic phase; og — group of 

 ovogonia. Redrawn from Coe, 1932a, fig. 9. Highly 

 magnified. 



oc' 



Figure 293. — Portions of two follicles of a bisexual gonad 

 of 4-month-old C. virginica. A — predominantly female, 

 and B — predominantly male follicle; gc — genital canal 

 lined with ciliated cells; oc — large ovocyte; oc' — young 

 ovocyte in spireme phase; spcl — primary spermatocytes 

 in spireme phase; spell — secondary spermatocytes; 

 spt — spermatides. Photographically reproduced from 

 Coe, 1932a, fig. 6. Highly magnified. 



Figure 294. — Two young ovocytes at spireme stage in a 

 mature ovary of C virginica. Redrawn from Coe, 1932a, 

 fig. 9. Highly magnified. 



EGG, SPERM, FERTILIZATION, AND CLEAVAGE 



325 



