vr DEVELOPMENT 369 



themselves : they are fertihzed immediately after being laid, 

 the male depositing spermatophores on the ventral surface of 

 the female's body just before oviposition. 



The process of segmentation of the oosperm presents 

 certain striking peculiarities. The nucleus divides repeatedly 

 (Fig. 91 A, nil), but no corresponding division of the pro- 

 toplasm takes place, with the result that the polyplast-stage 

 (p. 200), instead of being a heap of cells, is simply a multi- 

 nucleate body (compare p. 269). Soon the nuclei thus 

 formed retreat from the centre of the embryo, and arrange 

 themselves in a single layer close to the surface (b) : around 



I 







'""^?^'}i^''r<^^^- ■§ .'! ' 'k' 



'•■if"-- - t - <^y- 



.-.^i: 



.X 



Fig. 91. — Two stages in the early development of the Cr.ayfi^h. 

 In .\ the products of division of the nuclens (nit) are seen in the centre of tlie 

 yolh ; in B the nuclei have become arranged in a peripheral layer, each sur- 

 rounded by protoplasm, so as to form the blastoderm. (From Parker and 

 Hasu'ell's Zoology^ after Morin.) 



each of these protoplasm accumulates, the Central part of 

 the embryo consisting entirely of yolk-material. "We thus 

 get Si siiperficial segmentation, c\\a.xa.c\.mixA by a central mass 

 of yolk and a superficial layer of cells collectively known as 

 the blastodeim. From this the ectoderm and endoderm are 

 derived, the latter enclosing a relatively small enteron com- 

 municating with the exterior through a blastopore (p. 201). 

 Very soon the embryo becomes triploblastic. or three- 

 layered, by the budding off of cells from the endoderm 

 in the neighbourhood of the blastopore : these accumulate 

 between the ectoderm and endoderm, and constitute the 

 mesoderm. 



Pract. Zool. B 1; 



