VII 



DEVELOPMENT 



383 



coelome (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 immedi- 

 ately after being laid, the male depositing spermatophores 

 on the ventral surface of the female's body just before 

 ovi position. 



The process of segmentation of the oosperm presents 

 certain striking peculiarities. The nucleus divides 



FIG. 98. Three stages in the early development of the Crayfish, (x 10.) 

 In A the products of division of the nucleus (nu) 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.) 



repeatedly (Fig. 98, A, nu), but no corresponding division 

 of the protoplasm 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 the 



