42 EDWARD MCCRADY, JR. 



and figured a slight depression which, he says, for a long- 

 time marks the site of the blastopore. In a study of about 

 100 vesicles of the appropriate stages neither Hartman nor I 

 have been able to find this. 



The new polarity. As all of these cells arise in one-half of 

 the blastocyst, a polarization of the vesicle is again recog- 

 nizable. Accordingly, though it will involve a slight anticipa- 

 tion of details to be described more fully in the later sections 

 of this chapter, this seems to be the logical point at which to 

 discuss the possible relation of the polarity of the cleavage 

 stages to the polarity of the late blastocysts. 



Hartman 's opinion on this question was that the larger of 

 the first two blastomeres is probably the one which divides 

 more slowly, giving rise to a pole of large and primitive cells 

 which eventually become the ' embryonic area. ' The endoderm 

 mother-cells, he supposed, arose from this half of the vesicle 

 before the so-called 'embryonic area' was sharply defined. 

 Shortly thereafter, this area lagged so many generations 

 behind the more rapidly dividing 'trophoblast' that it came 

 to-be sharply demarcated from the latter. 



There are several difficulties with this interpretation. The 

 first is that it does not account for and is not compatible with 

 the loss of all visible polarity in the early unilaminar blasto- 

 cyst, which Hartman described. If the larger of the first two 

 blastomeres and its descendants are retarded in cleavage rate, 

 they will always remain larger than the rapidly dividing de- 

 scendants of the other blastomere which was smaller even 

 to begin with. 



This first objection, however, is obviously met by my own 

 studies of cell volume which showed that (as in the pig, 

 vide Heuser and Streeter, '29) the larger of the first two 

 blastomeres is actually the first to divide, and that it, not the 

 smaller one, gives rise to the pole of rapidly dividing cells. 

 This very clearly accounts for the temporary loss of size dis- 

 tinctions among- the cells in stage 9 ; for if the more rapidly 

 dividing cells are larger to begin with, they must necessarily 

 pass through a stage in which they are indistinguishable in 



