4 SIDNEY F. HAEMBR. 



sidering the great difficulty of making out anything of the 

 nature of the early stages except by means of sections. 



Barrois expressly states that the earliest stage to which he 

 succeeded in tracing his morulas with certainty is that repre- 

 sented in his pi. iii, fig. 3. This stage exactly corre- 

 sponds with the condition at which I have found the 

 embryos to be constricted off from the budding 

 primary embryo (cf. PI. II, fig. 11). Barrois was^ how- 

 ever, once successful in finding a cell, the egg nature of which 

 he considers uncertain (his pi. iii, fig. 1) ; and in another case 

 in finding what may have been an egg divided into twoblasto- 

 meres. It is not easy to say whether the former cell was 

 really an egg, or whether it was merely one of the giant-cells 

 described below. 



The rest of Barrois' account contains an erroneous history 

 of the later stages, which he himself was the first to correct 

 (2). I am compelled to doubt altogether Barrois' account 

 here given (not accompanied by any figures) of the supposed 

 occurrence of a process of segmentation of the egg, accom- 

 panied by the formation of an epibolic gastrula. In a later 

 paper (3) Barrois figures quite accurately the "morula" at 

 the stage at which it becomes independent (his pi. i, fig. 26), 

 although he wrongly supposes that the inner layer of cells 

 disappears in the later stages (his pi. iii, figs. 29, 30). 



Although Metschnikofi' (23, pi. xx, figs. 61 — 64) gives 

 admirable figures of the early embryos of Discoporella 

 radiata, the earliest stage observed by that author is the 

 stage at which the ''secondary" embryo becomes free from 

 the budding mass of embryonic cells. OstromnoJ0F (25) is no 

 more fortunate in elucidating the early history of the embryo 

 of Cyclostomata. 



I. Development of the Ovicell. 



This process, which takes place fundamentally in the same 



manner in all the species of Crisia which I have examined, 



has been to some extent described by Smitt (34), although 



most writers have paid little attention to the difference 



