PHYLUM ARTHROPODA 



557 



sftae (in the pleopods of the female by the sticky secretion of glands 

 occurring both on those appendages and on the segments them- 

 selves : they are fertilised immediately after laying, the male 



Fig. 441. — Keproductivc organs of Astacus fluviatilis. A, female ; B, male. oi'. oviduct ; 

 oe, its external opening ; ov. ovary ; t. testis ; u, unpaired posterior portion of gonad ; vd. vas 

 deferens. (From Lang's Cuimyarative Anatomy, after Huxley.) 



depositing spermatophores on the ventral surface of the female's 

 body just before oviposition. 



Development. — The process oi segmentation of the oosp'.n?i pre- 

 sents certain striking peculiarities. The nucleus (Fig. 442, A, mi,) 

 divides repeatedly, but no corresponding division of the protoplasm 

 takes place, with the result that the morula-stage, instead of being 

 a heap of cells, is multinucleate but non- cellular. Soon the nuclei 





.3- i- 



fiiS^'SVi 'i- V.-.: Bi« ■ ; 



'^■?^ 



■N''' 



, v;-'.r:;:«:,B:v^- 



•-' ■''--•■ ■^C,'W' ' 



Fig. 442.— Three stages in the formation of the blastoderm uf Astacus fluviatilis. nil. 

 nuelei ; yp, yolk-pyramids. (From Korsehclt and Hcider, after Morin and Reichenbach.) 



thus formed retreat from the centre of the embryo, and arrange 



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



"of these protoplasm accumulates, the central p;irt of the embryo 



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



