268 SOME RECENT 



Another very interesting result is the precise 

 point at which the power of developing an embryo 

 from a single blastomere ceases. Discussion has 

 taken place as to whether the failure is due to 

 quantitative or to qualitative considerations ; to an 

 insufficiency in amount of living matter, or to 

 incompleteness in its structure or composition. 

 Wilson is of opinion that the difficulty is mainly 

 a qualitative one, and has advanced arguments in 

 support of his view. Perhaps the most important 

 consideration in its favour is that so long as the 

 blastomeres are precisely alike i.e., up to the stage 

 with four blastomeres each one possesses the 

 power when isolated of developing into an 

 embryo ; while as soon as a distinction between 

 smaller and larger blastomeres appears t.e., at the 

 third cleavage this power ceases. 



This last consideration becomes still more 

 significant if we regard a Metazoon as comparable 

 to a colony of Protozoa, in which differentiation 

 between the component units, and consequent 

 mutual dependence, and the necessity for holding 

 together have become established ; and if further 

 we view the early development of a Metazoon as a 

 shadowing of the process by which this condition 

 was originally attained. So long as the cell units 

 or blastomeres remain identical, so long does each 

 one retain the power of independent existence and 

 development ; but as soon as differentiation is 

 established between one and another this power of 

 separate life ceases. This view, though extremely 

 suggestive, must however be regarded as entirely 



