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 



FIG. 450. Three stages in the formation of the blastoderm of Astacus fluviatilis. 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 

 the blastoderm(C ) . 

 Subsequently the 

 yolk itself under- 

 goes a process 

 of segmentation, 

 becoming divided 

 into radiating 

 yolk 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 : before 

 long, however, 

 these pyramids 

 fuse into an un- 

 divided mass of 



FIQ. 451. Ventral view of an embryo of Astacus fluviatilis, 

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

 cephalic lobe ; inv. invaginated area of blastoderm (endoderm 

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

 MacBride, after Reichenbach.) 



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 



