BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALS. 73 



proper is well indicated by the constriction at pd. Figure 16 shows 

 the normal size of the ovum before laying. 



b. Observations on Other Species. 



I have examined sections of the adult ovary in several other species, 

 and describe the condition in two of these, on account of the light it 

 throws on our conception of the fundamental structure of the ascidian 

 ovary. 



The first of these species is Styela montereyensis, 1 whose ovary 

 presents a structure similar to, but not identical with, that described 

 for Styela rustica by Floderus (p. 185-186). The ovaries are four 

 elongated tubes, two on each side, which open at their anterior ends 

 into the atrial cavity. The superficial wall is composed of a ciliated 

 epithelium, which connects with a germinative epithelium at the 

 lateral edges. The deep wall is much folded in an irregular manner, 

 and is composed of a thin pavement epithelium with occasional small 

 oogonia contained in its thickness. At times portions of the deep 

 wall, where these oogonia are crowded together, look a good deal like 

 germinative epithelium. I am convinced, however, that this is not the 

 case, for nowhere in the deep wall are oogonia of the smallest size 

 and undifferentiated ova, such as exist in the two lateral germinative 

 epithelia, encountered. The presence of these small oogonia outside 

 the germinative epithelium is due to the fact that the latter some- 

 times gives off its oldest oogonia after the younger ones, which are 

 thus carried out into the deeper wall before they are really ready, and 

 remain in it for a considerable period, while they are reaching the size 

 at which they project beyond the pavement epithelium. All the 

 older oogonia are situated on the deep side of this epithelium, presum- 

 ably attached to it by their follicular stalks, though these are but 

 seldom seen. In each of these four ovaries, then, we have essentially 

 the same structure that exists in Clavelina and Styelopsis, where the 

 ovary is single and situated in or near the sagittal plane. Further- 

 more, it is likely, though not proven, that these four ovaries are 

 developed from separate fundaments, for in a very much younger stage 

 (Plate 2, Fig. 12) , when the whole of the deep wall is made up of 

 the germinative epithelium, and the primordial ova are still dividing, 



1 This is one of the commonest of our California ascidians. It was first de- 

 scribed by Dall (71, pp. 157, 158) as Cynthia montereyensis, and subsequently by 

 Fewkes ('89, pp. 134, 135) as Clavelinopsis rubra. Ritter ('93, p. 39) placed it in its 

 proper genus, Styela. 



VOL. XXXV. — NO. 4. 2 



