96 bulletin: museum of comparative zoology. 



whole of the cytoplasm seems to be converted into that substance, for 

 the intervals between the yolk bodies are not occupied by the cytoplasmic 

 reticulum, — which at this stage takes a distinct though faint stain, — 

 hut by a confused mass of substance, all of which has the characteristic 

 hyaline appearance of the yolk bodies, in which, however, no structures 

 of definite shape have been formed. After the yolk bodies are fully 

 established, the absence of undifferentiated cytoplasm between them is 

 still more marked, for, like Davidoff, I have been unable to discover 

 trace of any other substance between them (Plate 5, Figs. 30, 31; 

 Plate 6, Fig. 50). When fully formed, the yolk bodies are of very 

 large size for ascidian material, and of various shapes. Those appar- 

 ently first formed are oval or spherical, while those of later develop- 

 ment are angular and occupy the spaces left by the first ones. 



Yolk formation begins when the ovum is about half grown (150 p 

 in diameter), and all of the cytoplasm is transformed into yolk some 

 time before the maximum size (about 300 /x) is reached. During the 

 growing period that follows the earliest stage at which all the 

 cytoplasm is converted into yolk, there is a little clump of yolk around 

 the germinative vesicle which has but indistinctly broken up into yolk 

 bodies (Plate 6, Fig. 49), and it may be from the periphery of this 

 clump that the new yolk bodies producing the increase in size are 

 formed. Ova possessing this clump of undivided yolk average about 

 200 fx, in diameter. 



Later this clump becomes completely broken up into yolk bodies 

 (Fig. 50). But none of these are spherical, as is usually the case in 

 other regions, all being more or less elongated in a radial direction. 

 It is probable that growth continues even after this central yolk has 

 broken up, for the range in size of ova in this condition is from 230 //, 

 to 326 p. It is impossible to determine the diameter of the mature 

 ovum, because, as soon as it begins to leave the ovary, it is immediately 

 deformed, and even in the pouch it is oval instead of spherical. But 

 it is not probable that the mature ovum would exhibit so much 

 variability. 



2. Germinative Vesicle. 



The chromatin in the young germinative vesicle is usually situated 

 next to the membrane, while in the centre no structural elements of 

 any kind can be detected (Plate 1, Fig. 4, ov'go.). The chromatic 

 granules are sometimes of uniform size (Fig. 4, ov'go.), but usually 

 there is one which is much larger than the rest, and destined to become 



