DEVELOPMENT 



383 



their products internally. Their cavities, lined by germinal 

 epithelium, represent part of the true ccelome (compare p. 

 374), and their ducts are organs of the same general nature 

 as nephridia, opening on the one hand into a coelomic 

 cavity, and on the other to the exterior. The ova, when 

 laid, are fastened to the setae on the pleopods of the female 

 by the sticky secretion of glands occurring both on those 

 appendages and on the segments themselves : they are 

 fertilised 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. 98. Three stages in the early development of the Crayfish. 

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

 yolk ; in B and C the nuclei have become arranged in a peripheral layer, each sur- 

 rounded by protoplasm, so as to form the blastoderm ; yp. yolk-pyramids. 

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



(Fig. 98 A, nu\ 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. 281). Soon the nuclei thus 

 formed retreat from the centre of the embryo, and arrange 

 themselves in a single layer close to the surface (B, c) : around 

 each of these protoplasm accumulates, the central part of 

 the embryo consisting entirely of yolk-material. We thus 

 get a superficial segmentation, characterised by a central mass 

 of yolk and a superficial layer of cells collectively known as 



