BANCROFT: OVOGENESIS IN DISTAPLIA OCCIDENTALIS. 81 



a. Evidence for the Intraovular Origin of the Test Cells. 



I have observed much of the evidence upon which Davidoff bases 

 his account, but do not consider it strong enough to establish his 

 contention. As a rule the wall of the young germinative vesicle is 

 perfectly smooth (Plate 3, Figs. 20-23, 13) , but in quite a number 

 of cases it is thrown into folds. A rather extreme instance of this 

 folding is shown in Figure 19 (Plate 3), which, however, represents 

 well the most characteristic feature, which is that the folds usually 

 occur at the ends of an oval germinative % vesicle. The long axis of the 

 germinative vesicle is often parallel to the edge of the knife used in 

 sectioning, so that in these cases the folding may be due to the shoving 

 of the section in cutting, but in other cases this cause cannot be in- 

 voked. Davidoff's figures represent the folding as being most pro- 

 nounced at the ends of the germinative vesicle, but not limited to that 

 region. In D. occidentalis, however, I found no cases of such exten- 

 sive folding as he has shown. 



Other structures that probably influenced Davidoff's conclusions, 

 and are sometimes associated with the folding, are the cytoplasmic 

 vacuoles near the germinative vesicle (Plate 3, Fig. 19). Oblique 

 sectioning and a faint stain, such as the borax carmine used by Davidoff 

 is liable to give, might combine to obscure the conditions so that the 

 vacuole would appear to be part of the germinative vesicle. It may 

 be that some of the nuclear evaginations seen by Davidoff were formed 

 in this way; but, even if what he figures are actually evaginations 

 from the germinative vesicle, it would not follow that they represented 

 a normal condition. Thus the presence of the vacuoles (Fig. 19) just 

 opposite to the mfoldings of the membrane seem to show that both are 

 artifacts due to some shrinking process. 



These vacuoles, however, are certainly some of the structures which 

 Davidoff has considered nuclear buds separated from the germinative 

 vesicle. In younger ova, where all of the cytoplasm takes a much 

 deeper stain, these vacuoles may have a stained periphery and thus 

 resemble a nucleus very closely (Plate 3, Fig. 22, vac). 



The intravitelline bodies, from which Fol ('83), Sabatier ('84), and 

 Eoule ('85) have derived both follicle and test cells, and Pizon ('93, 

 pp. 284-290) the test cells only, and the true nature of which 

 Floderus first elucidated, are also present in Distaplia (Plate 1, Fig. 7; 

 Plate 3, Figs. 22, 23; Plate 4, Fig. 28, cp. ia'vt.) , though they are 

 much smaller and occur less frequently than in Ciona. These bodies 



