CLEAVAGE. AND DIFFERENTIATION. 21 



work in embryology has been done upon forms in which the 

 cleavage is not known to be constant, and general conclusions 

 have been drawn which are plainly inapplicable to forms in 

 which the cleavage is constant and definite. Although it is 

 probable that there are forms which are intermediate between 

 those which show extreme constancy and those which manifest 

 extreme inconstancy of cleavage, yet the existence of two such 

 types of cleavage must be recognized, and, as it is desirable to 

 clearly distinguish between them, I propose to designate these 

 types by the terms determinate and indeterminate. This is to 

 be understood as applying only to the cleavage, for in its main 

 features and results the development of all animals is determi- 

 nate, that is, predictable. Even in cnidaria, echinoderms, and 

 vertebrates the general form of the cleavage is constant and 

 there appears successively a blastula, gastrula, larva, and adult 

 of determinate form and character. The question is whether 

 such determinism, which appears sooner or later in all cases, 

 applies to the individual blastomeres of the cleavasfe stages 



Determinate cleavage is both cojistant and differential. It is 

 more than constant, for in constant cleavage every blastomere 

 might be like every other (Driesch); it is more than differen- 

 tial, for differential cleavage might be of such a sort that it is 

 never twice alike (Whitman). It is the same as mosaic cleav- 

 age, but this name is not used because of the implication which 

 it involves as to the cause of differentiation; determinate cleav- 

 age does not necessarily imply "self-differentiation" of blasto- 

 meres, which is such an important part of Roux's "mosaic 

 theory." Cleavage is indeterminate when it is either inconstant 

 or non-differential or both. 



Among certain gasteropods ^ which I have studied the cleav- 

 age is of a highly determinate character as regards both the 

 history and destiny of individual blastomeres and the relation 

 of the cleavage planes and egg-axis to the future planes of 

 symmetry. The chief axis of the ovum is established before 

 fertilization, probably in the ovary, and it determines the 



1 Four species of Crepidula, Urosalpinx, Sycotypus, Fulgur, Tritia, Illyonassa, 

 and Bulla. 



