No. I.] THE EMBRYOLOGY OF CREPIDULA. 189 



and I believe that they are to be explained as a foreshadowing 

 of larval characters, just as bilateral cleavages are zLsually attri- 

 buted to a precocious development of adtilt characters . 



Wilson ('93) emphasizes the fact that bilaterality in cleavage 

 is an inherited character. This is undoubtedly true, but it is 

 also just as true that radiality in cleavage is an inherited char- 

 acter. It is possible to conceive of a radiality which would 

 be due merely to extrinsic forces and stresses, but this is not 

 the radiality of cleavage ; for so far as now known the latter 

 is characterized by a definiteness in the directions of division 

 and in the size and form of the resulting cells, which such 

 extrinsic forces are wholly unable to explain. 



It seems to me highly probable that all forms of cleavage 

 are truly inherited, just as certainly as the size and shape and 

 character of the egg or spermatozoon are inherited. The loose 

 character of the aggregate of blastomeres in Amphioxus, the 

 compact form of cleavage with its definite spirals in the annelid 

 or moUusk, the bilateral arrangement of the blastomeres in the 

 ascidian, all are ultimately due to the same thing, viz., the 

 structure of the germinal protoplasm. These peculiarities could 

 not be produced by extrinsic forces, they must come from 

 within; and, if I understand the word at all, this is just what 

 distinguishes heredity. On the other hand, certain minor 

 features in all these forms of cleavage are due to extrinsic 

 factors, and consequently the forms of cleavage, like all other 

 forms of the organism., are the resultants of the intrinsic and 

 of the extrinsic factors of development.^ 



(3) Significance of Bilateral Cleavages. — In the case of 

 bilateral cleavages the law of alternations or rectangular inter- 

 sections is violated more or less from the beginning ; and 

 likewise the principle of minimal contact surfaces is more or 



1 In a review of Wilson's work, Driesch ('95) criticises this very point in a 

 way with which I thoroughly agree. He finds the cause of all different kinds 

 of cleavage in the structure of the protoplasm, and hence concludes that one is 

 as truly inherited as the other. It seems to me that this conclusion differs radi- 

 cally from some of his earlier views concerning cleavage ; indeed, I am unable 

 to harmonize it with other expressions in this same paper, e.g., he says that there 

 can be no phylogenetic significance in the close resemblance between the cleav- 

 age in annelids and gasteropods because it has been mechanically produced (see 

 P- 195)- 



