XI 



PHYLUM ARTHROPOD A 



515 



Development. The process of segmentation of the oosperm pre- 

 sents certain striking peculiarities. The nucleus (Fig. 408, A, nu) 

 divides repeatedly, but no corresponding division of the protoplasm 

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



FIG. 407. Reproductive organs of Astacus fluviatilis. A, female ; B, male ; od. oviduct ; 

 re, ts external opening ; or. ovary ; t. testis ; u. unpaired posterior portion of gonad ; f<J. vas 

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



a heap of cells, is multinucleate but non-cellular. 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 each 

 of these protoplasm accumulates, the central part of the embryo 

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





FIG. 40S 



Three stages in the formation of the blastoderm of Astacus fluviatilis. nv, 

 nuclei ; y. p. -yolk-pyramids. (From Korschelt and Heider, after Morin and Reichenbach.) 



segmentation, characterised by a central mass of yolk and a super- 

 ficial layer of cells collectively known as the blastoderm (C). Sub- 

 sequently the yolk itself undergoes a process of segmentation, 

 becoming divided into radiating yjlk-pyramids(y.p.), each with its 

 base in contact with one of the cells of the blastoderm and its 



L L 2 



