XI 



PHYLUM ARTHROPODA 



557 



seta3 on 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 



B 



t 



OD 



FIG. 441. Reproductive organs of Astacus fluviatilis. A, female ; B, male. od. oviduct ; 

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

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



depositing spermatophores on the ventral surface of the female's 

 body just before oviposition. 



Development. --The process of segmentation of the oosperm pre- 

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

 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 



B 



FIG. 442. Three stages in the formation of the blastoderm of Astacus fluviatilis. nu. 

 nuclei ; yp. yolk-pyramids. (From Korschclt and Heider, 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 part of the embryo 

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



