344 



THE CRAYFISH 



LESS. 



protoplasm accumulates, the central part of the embryo 

 consisting entirely of yolk-material. We thus get a super- 

 ficial segmentation, characterised by a central mass of yolk 

 and a superficial layer of cells collectively known as the 

 blastoderm (c). 



On one pole an invagination of the blastoderm takes 

 place, giving rise to a small, sac, the enteron, which commu- 

 nicates with the exterior by an aperture, the blastopore. By 

 this process the embryo passes into the gastrula-stage, which, 

 however, differs from the corresponding stage in Polygordius 



Fie. 89. Three stages in the early development of the Crayfish. 



In A the products of division of the nucleus (mi) are seen in the 

 centre of the yolk : in B and c the nuclei have arranged themselves in a 

 peripheral layer, each surrounded by protoplasm, so as to form the 

 blastoderm. 



(From Parker and Haswell's Zooiogy, after Morin.) 



(p. 295) in the immense quantity of foool-yolk filling up 

 the space (blastocoele) between ectoderm and endoderm. 

 Very soon the embryo becomes triploblastic, or three-layered, 

 by the budding off of cells from the endoderm in the neigh- 

 bourhood of the blastopore : these accumulate between 

 the ectoderm and endoderm, and constitute the mesoderm. 

 Before long the blastopore closes, and a stomodseum and 

 proctodaeum (p. 296) are formed as invaginations of the 

 ectoderm which eventually communicate with the enteron, 

 forming a complete enteric canal. On each side of the mouth 



