420 CEPHALOCHORDA, AMPHIOXUS. 



actively, the difference between macromeres and micromeres has by this 

 time been established ; in fact the cells have begun to be specialised, 

 and have no longer the primitive indifference, the absence of differentia- 

 tion, which explains the developmental potentiality of the separated units 

 of the two-celled or four-celled stages. 



Somewhat similar experiments have been made by other investigators 

 on the developing ova of ascidians, sea urchins, &c. Specialisation of 

 segmentation cells appears to occur at different times in different animals, 

 but it is illogical to infer the absence of specialisation from the fact that 

 any of the first four blastomeres, let us say, can produce an entire embryo. 

 For specialised cells may retain a power of regeneration. 



