218 HUBERT DANA GOODALE 



Since the material at this point moves along these meridians to 

 the blastopore, it follows that the second plane of cleavage cor- 

 responds at least approximatel}^ to the position of the posterior 

 end of the neural folds. Accordingly, then, the second plane 

 of cleavage separates not anterior from posterior, but dorsal 

 from ventral, as Kopsch and Spemann claim. In the four-cell 

 stage then, there are two dorsal and two ventral blastomeres. 



If the second plane of cleavage happens to coincide with the 

 median plane of the embryo, then it follows that the first plane 

 separates a dorsal and a ventral blastomere. 



If an angle of 45° is made between the two first planes and the 

 median plane of the embryo, one of the first four blastomeres 

 will be wholly dorsal and one ventral, while each of the others 

 will be half dorsal and half ventral. Still other angles will pro- 

 duce correspondingly various degrees of distribution of the egg 

 material into the various blastomeres. 



The egg axis, according to the view I have adopted of the posi- 

 tion of the embryo on the egg, becomes not the dorso-ventral 

 axis of the embryo but the antero-posterior. Differences existing 

 in the position of the head of the embryo will modify this state- 

 ment slightly. In other words, the future embryo, in the narrower 

 sense, lies almost entirely in one half of the egg, and before gas- 

 trulation mostly above the equator. 



Since most of the work done to test the equipotence or toti- 

 potence of the egg has, tacitly at least, assumed that two of the 

 blastomeres of the four celled stage were anterior and two posterior, 

 instead of dorsal arid ventral, it follows that the living blastomeres 

 in those eggs in which the anterior blastomeres so-called, or a 

 corresponding amount of the material of the egg had been killed, 

 failed to develop beyond the gastrula stage, not because of the 

 injury, but because of an inherent inability to form an embryo. 

 Spemann, in his experiments, found that the ventral blastomere 

 remained alive a long time, but without differentiating. Evi- 

 dence is not lacking that others have found the same thing. 

 Thus, Hrdlicka describes ventral structures and more recently 

 Morgan and Lacquer describe posterior" half-gastrulae. The 



" More correctly, ventral. 



