336 The Oogenesis of the Tortoise 



condition would certainly suggest that it has something to do with 

 yolk-formation as is frequently asserted. It is often found in rather 

 small patches scattered throughout the cytoplasm, especially in the peri- 

 pheral zone. And this condition seems to be most frequent at about 

 that period in the egg's history when the true yolk-bodies arise. 



There, are, however, other facts which almost preclude the possibility 

 of this substance being converted into yolk-bodies directly. In the 

 first place, the yolk-bodies arise, first between the peripheral zone and 

 the subcuticular layer where the yolk-nucleus is rarely to be seen. In 

 the second place, the yolk-nucleus exists in the egg when the cytoplasm 

 is merely granular. The yolk-nucleus makes its appearance as soon as 

 the cytoplasm assumes its granular appearance and may be found up 

 to the time of true yolk-formation. 



I am, therefore, led to the following conclusion regarding the yolk- 

 nucleus: It is a hind of metaplasm (or archoplasm) arising in the 

 neighborhood of the germinal vesicle through the combined influence 

 of the nucleus and cytoplasm. From the place of its formation, it 

 diffuses or flows throughout the cytoplasm where it serves as a culture 

 medium of the living substance of the egg; in other words, it serves as 

 food. The true yolk-bodies are a secretion of the living substance of 

 the cytoplasm. 



The growth of the egg seems to be due largely to the growth of the 

 cytocenter, originally the centrosome. As this expands, the germinal 

 vesicle approaches more and more the periphery, and is consequently 

 greatly removed from the cytocenter formerly so near to it. It still 

 retains its relation to the cytocoel, and this is possible because the 

 peripheral zone becomes greatly thinned out owing to the expansion of 

 the cytocenter and the accumulation of yolk-bodies within the latter. 

 Eeference to the plates will make this clear. By comparing the differ- 

 ent regions of the cytoplasm in the earlier stages with the large eggs 

 represented in Plate VII, the region of greatest growth is easily seen 

 to be the central portion corresponding to the original sphere. The 

 region of greatest growth is also the region where the greatest amount 

 of yolk accumulates; hence the vegetative pole. 



Polarity of the Egg. — The polarity of this egg is marked from the 

 beginning and is determined by the relative position of the cytocenter 

 and the germinal vesicle. In the young oocyte, immediately after the 

 telophase of caryokinesis of the oogonium, the centrosome remains, as 

 already stated, at one pole of the nucleus, now the germinal vesicle. 

 The uniaxial feature of the spindle in that division remains in the 

 young oocyte, being determined, in this stage, as in later stages, by the 



