400 



INHERITANCE AND DEVEIOPMENT 



many later observers, which have shown that in the cleavage of anne- 

 lids, moUusks, platodes, tunicates, and many other animals, every cell 

 has a definite origin and fate, and plays a definite part in the building 

 of the body.^ 



;?2 s 



Fig. 182. — Half-embryos of the frog (in transverse section) arising from a blastomere of the 

 two-cell stage after killing the other blastomere, [Roux.] 



A. Half-blastula (dead blastomere on the left). B. Later stage. C. Half-tadpole with one 

 medullary fold and one mesoblast plate ; regeneration of the missing (right) half in process. 



ar. archenteric cavity ; c.c. cleavage-cavity ; ch. notochord ; ?«./. medullary fold ; m.s. meso- 

 blast-plate. 



In an able series of later works Whitman has followed out the sug- 

 gestion made in his paper of 1878, cited above, pointing out how 

 essential a part is played in development by the cytoplasm and insist- 

 ing that cytoplasmic preorganization must be regarded as a leading 

 factor in the ontogeny. Whitman's interesting and suggestive views 

 are expressed with great caution and with a full recognition of the 



1 Cf. p. 378. 



