DEVELOPMENT—YELK DIVISION. 205 
The answer to these questions is to be sought in the 
facts of individual and ancestral development. 
An animal not only is, but becomes; the crayfish is the 
product of an egg, in which not a single structure visible 
in the adult animal exists: in that egg the different tissues 
and organs make their appearance by a gradual process of 
evolution ; and the study of this process can alone tell 
us whether the unity of composition suggested by the 
comparison of adult structures, is borne out by the facts 
of their development in the individual or not. The 
hypothesis that the body of the crayfish is made up of a 
series of homologous somites and appendages, and that 
all the tissues are composed of nucleated cells, might be 
only a permissible, because a useful, mode of colligating 
the facts of anatomy. The investigation of the actual 
manner in which the evolution of the body of the crayfish 
has been effected, is the only means of ascertaining 
whether it is anything more. And, in this sense, deve- 
lopment is the criterion of all morphological speculations. 
The first obvious change which takes place in an im- 
pregnated ovum is the breaking up of the yelk into 
smaller portions, each of which is provided with a nucleus, 
and is termed a blastomere. In a general morphological 
sense, a blastomere is a nucleated cell, and differs from 
an ordinary cell only in size, and in the usual, though by 
no means invariable, abundance of granular contents ; 
and blastomeres insensibly pass into ordinary cells, as 
