No. I.] THE EMBRYOLOGY OF CREPIDULA. 191 



On the other hand, in most Echinodermata, Coelenterata, 

 and Vertebrata, no such definiteness in the history of the blas- 

 tomeres is known to exist. Of course the possibility remains 

 that in most, if not all, of these cases the cleavage is of just as 

 determinate a character as in the first class mentioned, and 

 that the denial of a definite prospective value to each blasto- 

 mere must rest upon the curious basis that no one has followed 

 a single blastomere through the development. I confess that 

 to me this possibility seems extremely probable. 



Under present circumstances, however, it would be unjusti- 

 fiable to classify all cleavage as equally determinate in character, 

 merely on the grounds of analogy with such cases as the anne- 

 lids and mollusks. There is some evidence that the extent of 

 predetermination differs in different cases (see Wilson, '93 and 

 '94) ; I propose, therefore, to classify all cases in which prede- 

 termination is not known to exist as indeterminate cleavage. 

 Such a classification is in many respects an unsatisfactory one, 

 and it can only be regarded as having a temporary value, but it 

 will serve to emphasize a distinction which in our present state 

 of knowledge we must recognize as existing. 



Most of the earlier experimental work in embryology was 

 done upon forms in which the cleavage is not known to be 

 determinate in character, and many general conclusions were 

 drawn which are not applicable to determinate cleavage. For 

 example, some of Driesch's conclusions have been too sweep- 

 ing ; no one who has ever studied such determinate forms of 

 cleavage as are exhibited by the annelids and the mollusks 

 could for a moment admit the truth of his earlier conclusion 

 ('93) : " By segmentation perfectly homogeneous parts are 

 formed capable of any fate." There is every ocular evidence 

 that in the cases referred to, the parts separated by cleavage 

 are not perfectly homogeneous, and under such circumstances 

 to assert that they are would be the climax of self-stultification. 



There is, I think, a fallacy in Hertwig's much-quoted dictum 

 ('92) : <' In consequence of the continuity of development, every 

 older cell group must arise from a younger cell group, and so 

 finally definite parts of the body from definite segment cells." 

 A true conclusion would be this: "And so finally definite parts 



