258 WATASE. [Vol. IV. 



One rational answer to our inquiry is possible ; viz. the proto- 

 plasmic structure of the egg which gives rise to one organism, 

 must differ from that of the egg which gives rise to another dif- 

 ferent organism, the differences between the two being relatively 

 as great as those which the two adult organisms display in their 

 anatomical relationships. 



If the similarities of two organisms must be attributed to the 

 corresponding similarities of the protoplasmic structure of the 

 fertilized ova from which they respectively arise, the source of 

 similarities in the latter must be sought for in the community 

 of their hereditary antecedents. Hence, one way to place the 

 doctrine of phyletic kinship of two or more organisms upon a 

 scientific basis, would be, if such a thing were possible, to demon- 

 strate the molecular or structural affinity of their germ-cells. 

 The embryological phenomena of a developing organism may 

 be expressed in the terms of protoplasmic metamorphosis. Two 

 organisms at the same stage of development would represent 

 the same stage of protoplasmic structure. The budding of a 

 new cell or the formation of a new organ would correspond to 

 the birth of a new phase in the course of the metamorphosis of 

 the original protoplasm of the Qgg. 



To turn to our main point, namely, the development of the 

 organism as first indicated by the cleavage of its protoplasmic 

 material. 



What is the cleavage of the ovum .-* What is accomplished 

 by it .-• Is it the mere sundering of material which has no more 

 reference to the future organization of the embryo than the 

 snowflakes bear to the size and shape of a future avalanche ? Or 

 is it a " histogenetic sundering" of material in which every step 

 in the process has a definite relation to the building up of the 

 future embryo .? That each step of cleavage has some definite 

 significance in relation to the organization of the adult or the 

 larva, at least in certain forms which have been most carefully 

 studied, there can be no question. Thus, in a certain Tunicate 

 already referred to, it has been observed that the nuclear sub- 

 stance of the ovum is divided, during the first cleavage, in such 

 a manner that one of the new nuclei, by its division, gives rise 

 to the right, and the other to the left side of the adult organism. 

 In another case, as in some worms, it has been maintained, that 

 the first division of the nucleus distributes the nuclear substance 



