EFFECTS OF CARBON DIOXIDE ON EGGS 363 



median, but in eggs examined after the division, we frequently 

 find the JNISt blastoinere lying outside of the median plane. In 

 these eases, the A and B derivatives are very asymmetrically 

 distributed over the dorsal portion of the embryo and it seems 

 that these cells push the MSt blastomere out of its normal posi- 

 tion. This is more apparent on models of the eggs, of course, than 

 in figures. The probable significance of this will be taken up later. 



Alany hundreds of eggs similar to those shown in figures 10 to 

 13 have been analyzed. The abnormal positions of the A and B 

 cells are retained, there is no shifting to form the rhombus, at 

 least it is not usually realized, the ectodermal cells derived from 

 the Si blastomere take up positions on almost any part of the 

 egg. Thus the bilateral symmetry of the embryo may be 

 completely lost and, what is for our study more significant, 

 the members of the ventral family, especially the MSt blasto- 

 mere, may be moved out of the median plane. 



The most striking results of the unequal distribution of the 

 chromatin between the A and B blastomeres is shown in figure 14. 

 Aside from the positions which these two cells have, we see that 

 one is dividing early and that it contains only a small number 

 of somatic chromosomes in the spindle. The mate, on the 

 other hand, shows no sign of division, and it will be noted that 

 it contains a very large nucleus (compare fig. 14 with fig. 6). 

 This result always follows the unequal distribution of the chro- 

 matin, apparently, and the early division of the one cell upsets 

 the cleavage rhythm of the Si derivatives. If the distribution 

 has been very uneven, then one of the cells may divide twice 

 before its mate cleaves. With a more equal distribution the 

 rhythm is not so markedly upset, but in any event the end result 

 is the same : the Si derivatives become scattered irregularly over 

 the surface of the embryo, the symmetry or balance of the em- 

 bryo is upset, and probably members of the ventral family are 

 pushed out of their normal positions, as was the case with the 

 egg shown in figure 13. 



The development of the embryos which showed a tetraster 

 in the Si blastomere, is extremely variable, both in the number 

 of cells formed by the division and in the distribution of the 



