318 The Oogenesis of the Tortoise 



At the close of the last division, the cells are arranged as seen in Plate 

 I, Figs. 10, 11. The chromosomes evidently become vacuolated, by the 

 secretion of a hyaline matrix, which separates the chromatin substance 

 into granules arranged on delicate fibres of linin, forming a network. 

 At first, all the nuclei thus formed are about equal in size, all being 

 spherical, and having the chromatin more abundant around the peri- 

 phery. The cytoplasm is relatively scarce, forming only a thin layer 

 around the nucleus of each cell. Comparing Figs. 10 and 11, it can be 

 seen that all the cells just divided, and before the nuclear reticulum is 

 fully formed, are very similar, each having a centrosome surrounded by 

 archoplasms, situated at one pole of the nucleus. In Plate I, Fig. 11, 

 on the other hand, only the central cell has an undoubted centrosome at 

 one pole. Even in this cell, the centrosome is not so distinct as pre- 

 viously, the archoplasm having apparently spread out along the sides of 

 the nucleus, forming a crescent. The centrosome is now a tiny granule, 

 occuping a clear globule, which is surrounded by a circle of larger micro- 

 somes. Evidently this is the resting condition of the sphere, the fibrils 

 being relaxed, and the microsomes of the peripheral ring separated from 

 the central granule, rendering the whole slightly more difficult to see. 

 In the peripheral cells, on the other hand, the slight quantity of cyto- 

 plasm surrounds the nucleus equally on all sides. But the nucleus in 

 these future follicle cells show, even in this early stage, a comparatively 

 large central body which resembles a nucleolus. It is difficult to believe, 

 however, that it is a nucleolus, since the true nucleoli develop much later. 

 I am inclined to believe that this is the centrosome of the preceding 

 stage. These cells, so far as can be seen, have a radial symmetry, pos- 

 sibly due to the position of the centrosome within the nucleus. The cen- 

 tral cell, the oocyte, on the other hand, shows the centrosome in the 

 cytoplasm at one pole, and hence has a more oval form. A distinct 

 polarity, in other words, exists here; and this seems to be due to the 

 relative position of the nucleus and centrosome respectively. I can see 

 no reason whatever, for doubting that this centrosome is the centrosome 

 of the dividing oogonia (Plate I, Fig. 11), and that the transition from 

 the condition existing in Fig. 10 to that of Fig. 11 is a complete trans- 

 formation, and a formation de novo of the centrosome in the central cell. 

 This centrosome is not a transient body, as the subsequent history of the 

 growing oocyte shows. This fact, too, can hardly be denied, namely: 

 That, first, the nucleus of the central cell or oocyte, now the germinal 

 vesicle, in its earliest stage, is derived directly from the chromatin of the 

 dividing oogonia, and hence is a direct continuation of the nucleus of the 

 oogonia ; and, second, that the cytoplasm, instead of being formed de novo 



