368 THEOPHILUS S. PAINTER 



mesoderm, and the cells of the stomodaeum. The Po cell, after 

 givmg off several generations of ectodermal and mesodermal 

 ceils, forms the primordial germ cells. The most important 

 points for us to remember are the following : During later develop- 

 ment the A and B cells grow over the dorsal and anterior end of the 

 body of the embryo. The EMSt and P2 cells he ventrally and 

 posteriorly and form the most important organs of the body. 

 Nearly all of the posterior part of the body of the young worm 

 comes from the Pi derivatives. 



We can now turn to the question of the relation of the normal 

 cleavage of the treated eggs and the worms which resulted from 

 them. 



Since a certain percentage of eggs always developed in a per- 

 fectly normal fashion during cleavage, it is clear that the fully 

 developed normal worms, such as shown in figure A, arose from 

 this source. 



It is equally clear that the masses of totally disorganized cells 

 which one finds in figure C are due, in part at least, to the forma- 

 tion of the tetraster in the Si blastomere, with the subsequent 

 abnormal development. No doubt other sources contributed 

 to this class of embryo. 



The embryos in which the posterior end is only partially dif- 

 ferentiated are undoubtedly to be traced to the eggs with the A 

 and B cells lying in abnormal positions, but the details of how 

 this condition affected the later development are uncertain be- 

 cause we have so little knowledge in how far the later shifting 

 of the blastomeres in Ascaris is due to internal organization, and 

 in how far to simple mechanical relations, such as mutual pres- 

 sure, etc. Admitting this uncertainty at the start, we may give 

 a very simple explanation which appears to agree with all of the 

 observations recorded. 



A glance at figure I will show that the derivatives of the P] cell 

 form a sort of half keel on the ventral and posterior end of the 

 embryo, and the ectodermal cells, (derivatives of the Si) by their 

 division form a more or less symmetrical covering for this. The 

 works of Boveri ('09) and Miss Stevens ('09) have shown that 

 this ventral keel may take place when the A and B derivatives 



