THE FORMATION OF A BLASTODERM. 207 
yelk, and fabricate the body of the embryo. This process 
is termed partial or incomplete yelk division. 
The crayfish is one of those animals in the egg of 
which the yelk undergoes partial division. The first 
steps of the process have not yet been thoroughly worked 
out, but their result is seen in ova which have been but 
a short time laid (fig. 57, A). In such eggs, the great 
mass of the substance of the vitellus is destined to play 
the part of food-yelk ; and it is disposed in conical 
masses, which radiate from a central spheroidal portion 
to the periphery of the yelk (v). Corresponding with the 
base of each cone, there is a clear protoplasmic plate, 
which contains a nucleus; and as these bodies are all 
in contact by their edges, they form a complete, though 
thin, investment to the food-yelk. This is termed the 
blastoderm (bl). 
Each nucleated protoplasmic plate adheres firmly to 
the corresponding cone of granular food-yelk, and, in all 
probability, the two together represent a blastomere ; 
but, as the cones only indirectly subserve the growth of 
the embryo, while the nucleated peripheral plates form 
an independent spherical sac, out of which the body of 
the young crayfish is gradually fashioned, it will be con- 
venient to deal with the latter separately. 
Thus, at this period, the body of the developing crayfish 
‘is nothing but a spherical bag, the thin walls of which are 
composed of a single layer of nucleated cells, while its 
cavity is filled with food-yelk. The first modification 
