370 THEOPHILUS S. PAINTER 



posterior end, coming from the other end of the keel and more or 

 less independent of the derivatives of the Si cell, would be normally 

 formed, since all the necessary elements were present. It is 

 not to be expected that simply the MSt cell w^ould be affected. 

 No doubt the failure of this cell to take up the proper position 

 causes the whole anterior end to be disorganized, and when the 

 A and B derivatives were very irregularly distributed (when they, 

 for example, lay on top of the P2 cell without touching the EMSt, 

 and I have observed such cases) the disorganization probably 

 extended to the posterior end. It seems likely that the degree 

 to which the posterior end was differentiated is to be correlated 

 with the positions which the A and B derivatives took, and thus 

 we have a series of stages from worms which are seven-eighths 

 normal, to worms in which only the stump of the posterior end 

 is differentiated. 



In this way we are able to explain the production of the half 

 embryos following the treatment of the eggs with CO2. There 

 is, of course, another way of explaining their production, but this 

 has not been advanced because it did not harmonize, as it seemed 

 to me, with the facts which have been discovered by Boveri and 

 his students. We have assumed that the ectodermal cells com- 

 ing from the Si blastomere, were more or less indifferent in their 

 nature. This is indicated by the normal development, for the 

 cells divide rhythmically and form the general ectodermal cover- 

 ing for the anterior end of the body. In contrast to these, the 

 cells of the ventral family (Pi derivatives) divide very irregu- 

 larly and possess a high degree of specificity, that is, one forms 

 the entoderm, another the mesoderm, or primodial germ cells, 

 and so on. Were we to attribute specificity to the ectodermal 

 cells, then the explanation for the half embryos would be that 

 they were formed since the cells destined for the anterior end 

 were scattered and this part of the embryo was undifferentiated. 



The latter view does not seem tenable since all work points 

 to the indifferent nature of the Si derivatives. In any event, 

 however, the production of the half embryos is to be traced to 

 the abnormal positions which the A and B blastomeres take in 

 the 4-cell stage. 



