BANCROFT : OVOGENESIS IN DISTAPLIA OCCIDENTALS. 95 



yolk is green, so that, in the living embryo, they can be detected 

 easily, floating in the space between the embryo and the chorion, and 

 are seen to be left behind at the time of hatching. The process must 

 rather be considered as entirely degenerative, perhaps a futile attempt 

 to conform to the changed conditions and protect the cell from the 

 disastrous results of its active metabolism, by means of which the 

 ovum is nourished. It seems well to emphasize the intensity of this 

 metabolism. 



V. Ovum. 

 1. Cytoplasm. 



As is usual in ascidians, the cytoplasm of the young Distaplia ova is 

 filled with rather large granules that take a nuclear stain with avidity 

 (Plate 1, Fig. 4; Plate 3, Figs. 21, 22), but while the test cells are 

 being formed the stain becomes fainter and fainter, until it almost 

 vanishes. Together with the decrease in stainability, there is a 

 decrease in the size of the granules (Plate 3, Figs. 23, 13), until at 

 last they become so inconspicuous as to be negligible quantities 

 (Plate 4, Fig. 27). In later stages again they appear to become 

 more marked, but it is difficult to correlate the changes with the size 

 of the ova, for there is so much individual variation. It is clear, 

 however, that after the cytoplasm has ceased to stain deeply, good 

 preservation will often enable one to detect a cytoplasmic network, 

 the nodal points of which give the appearance of granules (Plate 3, 

 Fig. 26; Plate 4, Fig. 28). This condition comes out more distinctly 

 in the later stages figured, but indications of the same thing are seen 

 earlier, and the reticulum probably exists even where it is obscured 

 by the deep stain. The network persists up to the time of the forma- 

 tion of the yolk bodies, and is present in the central part of the ovum 

 when the periphery already contains much yolk (Fig. 28). 



In Distaplia magnilarva DavidofF ('89, p. 155) did not find any 

 stages in the process of yolk formation, and hence concludes that the 

 yolk bodies are formed simultaneously throughout the whole extent of 

 the ovum. In our species, however, stages are often encountered 

 in which the yolk bodies are present at the periphery only (Fig. 28). 

 Here the largest yolk bodies are scattered amongst the test cells, and 

 as the germinative vesicle is approached they become progressively 

 smaller, until near the centre there is a gradual transition into the 

 cytoplasmic reticulum. However, where the yolk is forming the 



