BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 71 



genera- assume. In Distaplia occidentalis this arrangement of the 

 oogonia, and even their existence there, has reference to the phylogeny 

 of the genus rather than to present usefulness, for the usual number of 

 eggs laid hy a zooid is two, or at most three, and since in the adult 

 there is no provision for budding, all the younger oogonia must degen- 

 erate with the zooid and go to waste. 



Thirdly, there is the stalk tissue (Figs. 5, 6, Us. pd.), which is 

 directly continuous with the gorminative epithelium or its peripheral 

 membrane, and also by means of the follicular stalks with the follicles 

 of all the older ova (Plate 3, Figs. 14, 15, Us. pd.). This, in its 

 typical condition (Figs. 5, 15), is a cylindrical epithelium with 

 rather irregular oval nuclei and a thick, structureless basement mem- 

 brane on its peripheral surface. The stalk tissue surrounds a cavity 

 (Fig. 5, lu. pd.) that is usually somewhat larger than the true cavity 

 of the ovary, with which it connects by a narrow stalk. This stalk 

 joins the rest of the ovary in the region of the germinative epithelium, 

 so that a series of cross-sections presents exactly the same appearance as 

 in Fragaroides. Posteriorly the deep wall is composed of a continuous 

 germinative epithelium; about the middle of the epithelium the cavity 

 of the stalk tissue connects with the cavity of the ovary proper and 

 divides the germinative epithelium so that in a cross-section of this 

 region it looks like a paired structure (Plate 1, Fig. 5). Anteriorly 

 the entire centre of the deep wall is made up of germinative epithelium 

 as it is posteriorly. As a result of this structure, it is seen that the 

 stalk tissue does not connect with the flat pavement epithelium com- 

 prising the wall of the oviduct. The cavity of the stalk tissue extends 

 posteriorly behind that of the ovary proper, and has the follicles 

 attached to it in such a way that the oldest ovum is usually the most 

 posterior one. From this account it follows that the ovary, though 

 not a paired organ, is a symmetrical one ; but even this is not always 

 the case. Occasionally the stalk tissue, instead of being inserted near 

 the centre of the deep wall, joins the latter near one side, so that it 

 connects with the superficial wall on one side and the germinative 

 epithelium on the other. 



The question as to whether in Distaplia occidentalis there is a 

 limiting membrane which is distinct from the germinative epithelium, 

 seems to be answered negatively. The condition mentioned above, 

 and represented in Figures 5, 6, and 7, where the ova project strongly 

 into the cavity of the ovary, seems to indicate that the germinative 

 epithelium is bounded peripherally by a limiting membrane. 



