EFFECTS OF CARBON DIOXIDE ON EGGS 



367 



cycle, these two cells divide in planes at right angles to each other 

 (fig. E), and furthermore, as is well known, the chromatin of the 

 Si cell undergoes a process of "diminution" while the Pi cell 

 retains the elongated chromosomes. The result of the division 

 is four blastomeres which form a T-like figure (fig. F.) . Following 

 this, the cell marked P2 shifts around until it comes in contact with 

 the B blastomere, forming in this way a rhombus. Up to this 

 time, one can not speak of an anterior and posterior end of the 

 embryo, but after the shifting, these parts are marked out. The 



Text figures E to I 



anterior end lies to the right in the figure, that is, at the A and 

 EMSt side, while the posterior end is indicated by the P2 blasto- 

 mere. The A and B blastomeres lie dorsally, as in the figure, the 

 P2 and EMSt ventrally, the median plane of the embryo being 

 parallel to the paper and passing through all four blastomeres. 

 The A and B cells now divide in a plane approximately at right 

 angles to the median plane (fig. G), while the P2 and EMSt 

 cells divide in the median plane (fig. I) . The A and B cells give 

 rise to the ectoderm covering the dorsal and anterior end of the 

 body. The EMSt cell will give rise to the entoderm, part of the 



