No. I.] GERM-LAYERS IN" CLEPSINE. 



7 8 



127 



Diagrams of the four-cell stage of Clepsine and Rhy7tcheli7iis (^Euaxes') to show the 

 relations of the cleavage-planes to the 7nedian plane of the embryo. 



DiAG. 7. — Clepsine. The axis of the embryo bisects two opposite macromeres 

 {c and x). 



DiAG. 8. — Rhynchelmis (constructed after Vejdovsky). The embryonic axis here 

 bis'^cts opposite macromeres, but stands at right angles to the position shown in 

 DiAG. 7. 



a, c, b, X, the four macromeres. 



«/, the polar disc of hyaline protoplasm, which marks the posterior macromere. 



I - I, first cleavage-plane; II - II, second cleavage-plane. 



planes in Rhyncliclmis is the reverse of that in Clepsine. That is, 

 if the first and second cleavage-planes in Rhynchelmis correspond 

 respectively, to the second and first in Clepsine, there will be 

 a complete correspondence of macromeres and axial relations 

 in the two eggs. The appearances are so strongly in favor of 

 the identity of the posterior macromeres, that either alter- 

 native appears to me more acceptable than the conclusion 

 that there is a radical difference in axial relations that bear all 

 the outward marks of being absolutely identical. I shall pro- 

 ceed on the assumption that the difficulty just considered arises 

 from an error of observation, and that the relations between the 

 median plane of the embryo and the cleavage-planes are pre- 

 cisely the same in both cases. Future observation must deter- 

 mine whether this position is well chosen. 



The cleavage of the posterior macromere is described by 

 Vejdovsky in the following words : — 



" In den nachfolgenden Vorgangen spielt die hintere Makro- 

 mere die wichtigste Rolle, in der, wie bemerkt, das Protoplasma 



