138 BIOLOGICAL LECTURES. 



A re-investigation of the prototroch focussed upon these dis- 

 crepancies would be of great value in determining the trend of 

 the whole mass of evidence, whether it is for or against the 

 doctrines of the homology of cleavage cells, of the significance 

 of cleavage as a factor in differentiation, and of the preorgani- 

 zation of the egg itself. 



