THE FORMATION OF A BLASTODERM. 207 



j^elk, and fabricate the body of the embiyo. This process 

 is termed partial or incomplete j^elk division. 



The crayfish is one of those animals in the egg of 

 which the 3'elk 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-3^elk ; 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 (hi). 



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 



