312 McMURRICH. [Vol. IV. 



sky {'75) found the method to be as follows: "This phenome- 

 non (the segmentation) is quite regular, — i.e. the egg divides 

 into two spherules, then into four, and so on. There results 

 from this segmentation a mass of cells without the formation of 

 that central cavity which is termed the segmentation cavity. 

 After the segmentation is completed, the embryo covers itself 

 with vibratile cilia, and begins to swim." A Sterrula is conse- 

 quently formed in this case likewise, though from the descrip- 

 tion quoted it is impossible to state the exact manner in which 

 it arises. 



Before proceeding to compare these forms with Metridimn, it 

 will be necessary to mention certain cases in which an invagi- 

 nate gastrula has been stated to occur. These are a form 

 related to A. inesembiyajithemum and Ceriant/ms, according 

 to Kowalewsky ('75 and '73), and A. equina, according to 

 Jourdan ('80). I regret exceedingly that Kowalewsky's impor- 

 tant paper is inaccessible to me, and I have been obliged to rely 

 on the abstract given in Hoffman and Schwalbe's Jahresbe- 

 richt, and on the translation of a portion of the original paper 

 by Giard (Kowalewsky, '75). As for the figures, I have been 

 able to see only those reproduced by Mark ('84), in Pis. XI and 

 XII of the Selections from Embryological Monographs. Fig. 

 27 of PI. XI, representing the gastrula of A. mesembryanthe- 

 mwn {?), is very similar to that given by Jourdan from A. equina, 

 and it certainly looks like an early stage of invagination. 

 Nevertheless, it seems to me, in view of what I have described 

 for Metridinni, that there is a very great chance that both 

 Kowalewsky and Jourdan have been mistaken in their interpre- 

 tation of what actually occurs. I have found in Metridiiim 

 what I considered malformations, which approach very closely 

 to what these authors describe and figure. I find in my notes 

 the following statements : " In many {i.e. swimming blastulas) 

 a depression could be observed upon one side, giving rise to 

 a slight projection into the blastocoel. The invagination {i.e. 

 depression) appears to occur mostly at the side. Never saw 

 one at end." I have drawings of optical sections of this which 

 present an appearance not very unlike what Kowalewsky and 

 Jourdan have figured. It must be noticed, too, that neither of 

 these authors were able to trace directly one of these stages 

 into the fully formed gastrula stage. Jourdan gives as the sue- 



