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THE AMERICAN NATURALIST [Vol. XLIV 



set of chromosomes is present in the embryo — two of 

 each kind of chromosomes — and this fact is of signifi- 

 cance in heredity. 



Boveri has added further evidence in favor of his 

 conclusion from an experiment in which normally fer- 

 tilized eggs are put under pressure just as the cleavage 

 is about to appear. The cytoplasm division often fails 

 to take place. A single cell may sometimes contain two 

 nuclei and such cells not infrequently later form poly- 

 asters. These may cause inequalities in the distribution 

 of the chromosomes, and the abnormal development that 

 sometimes follows can be explained in the same way as in 

 the case of the dispermic eggs. Boveri asks what can 

 these cases have in common unless it is the inequality in 

 di-t 1 • i 1 > 1 1 1 1 o 1 1 of the chromosomes. 



Driesch has argued that, since in the normal develop- 

 ment the plane of bilaterality corresponds with the first 

 (Boveri) or second (Driesch) plane of cleavage, the 

 three-fold or four-fold types may fail to produce this 

 effect at the right moment. But it is not evident, even 

 if it is true that a bilaterality exists in the egg, that the 

 embryo might not still produce it independently of the 

 cleavage. In the case of the four-fold type an opportu- 

 nity is, in fact, furnished for the normal relation to ap- 

 pear, yet this type produces fewer normal embryos than 

 does the three-fold type. Moreover, the development of 

 symmetrical embryos for the one-half and one-fourth 

 blastomeres shows- that the egg has remarkable regula- 

 tory powers in this regard. Again radially symmetrical 

 embryos have been produced by ITerbst in lithium solu- 

 tions, yet these do not appear in embryos from disper- 

 mic eggs. 



This evidence goes far towards establishing in some 

 form the probability of Boveri 's argument. It seems to 

 me more cogent and convincing than that brought for- 

 ward by his opponents. It does not, I think, prove that 

 the chromosomes are entirely unlike and does not, ob- 

 viously, prove that each character of the embryo is 

 located in a particular chromosome. But the evidence 



