EFFECTS OF CARBON DIOXIDE ON EGGS 369 



are absent (as when the Si is killed by ultra-violet light). This 

 indicates a high degree of internal organization in these cells. 

 Here it may be remarked that in these experiments there were no 

 ectodermal cells which might move the members of the ventral 

 family out of the median plane. Were, however, the A and B 

 derivatives present but in positions which would destroy the 

 symmetry of the embryo (for example, were they lying only on 

 one side of this keel, and I have observed cases which approached 

 this) to unbalance the system, so to speak, then it seems very 

 probable that some of the ventral cells would be moved out of 

 the median plane. 



In the later development of the eggs in which the A and B cells 

 occupy abnormal positions, we seem to see this unbalancing tak- 

 ing place. Quite frequently in such eggs, the majority of the 

 A and B derivatives lie on one side of the keel, and in the later 

 stages, as shown in figure 13, one of the blastomeres (the MSt 

 in this case) is moved out of the median plane. Prior to the 

 division, the EMSt blastomere lies in the median plane and 

 even in the metaphase, it holds this position. I have noted only 

 one or two exceptions to this. After division, however, the MSt 

 cell is frequently found lying out of the median plane, and the 

 most probable explanation is that it has been moved out of this 

 plane by the overlying ectodermal cells. It may well be that 

 other causes were operating to produce the same end effect. In 

 a few cases, the EMSt blastomere divided in an abnormal plane, 

 figure N shows such a case, or, disorganization may have come 

 later. 



As cleavage went forward in these eggs, the A and B deriva- 

 tives formed the cleavage cavity by mutual pressure and gas- 

 trulation took place. It was not until organ formation began 

 that the effects of the shifting of the MSt cell could be observed. 

 Looked at from a theoretical point of view, since the MSt blasto- 

 mere forms the cells of the stomodaeum (after several divisions) 

 we should expect that were this cell pushed out of its normal 

 position, the resulting embryo would lack this part. The em- 

 bryos in which the anterior end is disorganized but in which the 

 posterior end is normal, seem to fulfill these expectations. The 



