No. I.] GERM-LAYERS IN CLEPSTNE. 119 



I have called the second period in Clepsine. This period is 

 initiated by two events, — ( i ) the cleavage of the posterior 

 macromere, and (2) by the appearance of two small cells 

 beneath the micromeres. BUtschli directs his attention almost 

 wholly to the second of these events, and, with the exception 

 of a few incidental remarks, completely ignores the first. 



The posterior macromere first divides into two equal parts, 

 one of which occupies nearly a central position on the lower 

 face of the egg, while the other lies behind and slightly above, 

 abutting against the posterior edge of the ectodermic quarter- 

 foil. It will be remembered that similar relations are constant in 

 the corresponding stage of Clepsine, with the single difference 

 that the posterior of the two segments (the neuro-nephro- 

 blast) is usually only about one-half the size of the other. 

 It is a fact of considerable importance to us that the second 

 period is introduced in both cases in the same remarkable 

 manner {i.e., by the cleavage of one macromere), for we are 

 thus assured that events are still moving on in parallel courses. 

 I am quite confident that the two segments resulting from the 

 division of the posterior macromere will be found to correspond 

 to the neuro-nephroblast and the primary mesoblast of Clepsine, 

 and I shall henceforth designate them by these names. One 

 fact only, noticed by BUtschli, throws a little doubt on the 

 identification of the neuro-nephroblast. This segment buds 

 off two micromeres behind the four already formed at the 

 animal pole, and the six micromeres of this stage are arranged 

 in two longitudinal rows.^ I have seen nothing in the ^^^ of 

 Clepsine comparable with the last two micromeres, but I have 

 shown (No. I, p. 54, x'^) that two ectoblastic micromeres arise 

 in a little later stage, not directly from the neuro-nephroblast, 

 but from the immediate products of this segment. 



The neuro-nephroblast next divides by a sagittal cleavage 

 into two equal parts, which are probably the equivalents of the 

 cells marked x^ (Fig- 31, PI. XII.) in my first paper; and 

 then follows the cleavage of the primary mesoblast into two 

 equal parts, bilaterally disposed, and corresponding in position 

 and origin to the two mcsoblasts of Clepsine. According to 

 Robin (No. 5, p. 150) the primary mesoblast divides before 

 the neuro-nephroblast, in the same order as in Clepsine. 



' Precisely like the "mesomeres" in Rhynchelmisi. 



