544 



ZOOLOGY 



SECT. 



Development. The process of segmentation of the oosperm pre- 

 sents certain striking peculiarities. The nucleus (Fig. 450, A, nu) 

 divides repeatedly, but no corresponding division of the protoplasm 

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



nu 



nu. 



FIG. 450. Three stages in the formation of the blastoderm of Astacus Suviatilis. nu 

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



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 

 segmentation, characterised by a central mass of yolk and a 



superficial layer 

 of cells collec- 

 tively known as 

 theblastoderm(C). 

 Subsequently the 

 yolk itself under- 

 goes a process 

 of segmentation, 

 becoming divided 

 into radiating 

 y oik pyramids 

 (y.p.), each with 

 its base in con- 

 tact with one of 

 the cells of the 

 blastoderm and 

 its apex pointing 

 to the centre of 

 the egg : beloiv 

 long, however, 

 these pyramids 

 fuse into an un- 

 divided mass of 

 yolk. 



The first indications of the future Crayfish take the form of 

 thickenings on what will become the ventral surface. There are 

 at first five of these thickenings two anterior, the. head-lobes 



lh.abd 



inv. 



FIG. 4 51. Ventral view of an embryo of Astacus fluvia tills, 

 gastrula stage, in order to show the ventral pinto, c.l. 

 cephalic lobe ; inv. invaginated area of blastoderm (fiidi>cii-i m 

 disc) ; th. abd. thoraeieo-abdominal rudiments. (From 

 MacBride, after Reichenbach.) 



