BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 75 



epithelia to the whole ovary, but the case of the deeper lobes of the 

 Chelyosoma ovary shows that its applicability is limited to ovaries, or 

 their parts, that have a superficial position. Stated in its most general 

 form, then, the law of growth within the ascidian ovary of the regu- 

 lar type seems to be that: 



In general, in ovaries or parts of ovaries having a superficial position 

 and bilateral symmetry, whether the germinative epithelium is double 

 or not, development takes place from the median line of the superficial 

 wall towards the deeper parts of the animal. 



Another general question is whether the Clavelina type of ovary, 

 with two separate germinative epithelia, should be considered more 

 primitive than that of Distaplia and Fragaroides, where a single germi- 

 native epithelium occupies the greater part of the deep wall. It seems 

 to me that the condition in Distaplia is more primitive, for it occurs 

 in smaller and simpler species, and is in itself a simpler condition. 

 Furthermore, this contention is borne out by the young Styela ovary 

 (Plate 2, Fig. 12), where the germinative epithelium occupies the 

 whole deeper wall of the ovary, though it is double in the adult. It 

 is, however, possible that both the condition in Fragaroides and Styela 

 may be the result of a secondary fusion; and Julin's work on Stye- 

 lopsis, where the germinative epithelium is formed from two single 

 rows of cells extending along the lateral edges of the ovarian funda- 

 ment, and proliferating primordial ova and oogonia towards the middle 

 line, favor this view. In Styelopsis the two epithelia thus formed do 

 not come near enough to fuse, but it may be that they do in other 

 species. In Distaplia, however, there is certainly no fusion, for 

 the germinative epithelium is single from the first. There is about 

 the same number of oogonia in the undifferentiated bud that there is in 

 the adult, so that the whole ontogeny of these cells consists in rear- 

 rangement and growth. A few of these oogonia commence to increase 

 in size at a very early period, and, as these are usually near the middle 

 of the deeper wall, it is here that the first follicle and consequently 

 the stalk tissue is formed. But we should expect that occasionally an 

 ovum near the edge would commence to grow first, and accordingly we 

 actually do find that in a few cases the stalk tissue is attached near the 

 lateral edge of the ovary. In fact, from the study of the development 

 in Distaplia, one gets the impression that the bilateral symmetry is 

 accidental , depending upon the fact that usually the first ovum to grow 

 will be somewhere near the centre of the deep wall. But that this 

 cannot be true for other species, and may not be true in this one, is 



