XI 



PHYLUM ARTHROPOD A 



515 



Development. The process of segmentation of the oospcrm pre- 

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

 divides repeatedly, hut no corresponding division of the protoplasm 

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



l-'i'.. 407. Reproductive organs (if Astacus fluviatilis. A, female; li, male; mi. oviduct ; 



ve, ts external opening ; m-. ov;iry ; t. testis ; u. unpaired posterior iiortion of gonad ; vd. vas 

 deferens. (From Lang's Comjinmtic, .4n<i.tn,nii, 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 

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



tut 



!'!'. 40S. Three stages in the formation of the blastoderm of Astacus fluviatilis nu, 

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



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

 ficial layer of cells collectively known as the U(.i.*l<><l<'ni> (( '). Sul>- 

 sequently the yolk itself undergoes a process of segmentation, 

 becoming divided into radiating yolk-pyramids(y.p.\ each with its 

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



L L 2 



