627 



has not been able to show one essential point of resemblance between 

 these half-eggs and embryos of meroblastic eggs. He himself has poin- 

 ted out many difference which render such a comparison worthless. 

 Apparently the only similarity that Hertwig has been able to 

 find is that in the later stages of development the dead or injured 

 half of the egg operated upon rotates below on account of its greater 

 weight while the lighter embryo lies above. Hertwig assumes, but 

 does not show, that the open posterior end of his embryos is com- 

 parable to a blastopore rim. 



It is by no means clear to me however what the true blastopore 

 is in these half forms. Nor do 1 understand Schultze's account of 

 the blastopore in the double embryo and its method of closure. The 

 small embryos are in both experiments under very peculiar conditions 

 and must be studied further before we can understand where the 

 blastopore lies and how it closes. 



Finally there are certain general problems in connection with this 

 experiment of killing one of the first two blastomeres, that must be 

 considered. Whether the following interpretation be true or false, it 

 need have no bearing on the preceeding criticisms which are based 

 on the direct evidence of experiment. 



The results show, I think, that the phenomena of half or whole 

 development of an embryo from one of the first two blastomeres is 

 entirely a protoplasmic phenomenon. The results have nothing what- 

 soever to do with a qualitative division of the egg at the first clea- 

 vage or with a later postgeneration. Whether we get a half-or a 

 whole-embryo will depend upon the subsequent arrangement of the 

 protoplasm in the uninjured blastomere and upon the relation of the 

 protoplasm of the uninjured and injured halves. If the egg is turned 

 after one blastomere has been injured, so that a rotation of the con- 

 tents of the uninjured blastomere takes place then a whole embryo 

 tends to develop. The completeness of the development will depend 

 upon the extent of the rotation. 



If the egg after the operation retains its normal position then a 

 half larva will develop. 



A further possibility must also be considered viz when the injury 

 to one side is very extensive so that the injured blastomere no longer 

 retains its relation as a half to the uninjured blastomere then it 

 may happen that even here a whole embryo may develop irrespective 

 of the position of the egg after the operation. Only a much more 

 complete series of experiments than I have been able to make will 

 determine whether this latter possibility is ever realized in the egg. 



