558 



ZOOLOGY 



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 yolk pyramids (y.p.), each with its 

 base in contact 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 undivided 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 

 (Fig. 443, K), on which the eyes subsequently appear ; two some- 

 what further back, the thoracico-abdominal rudiments (T A) \ and 

 one, posterior and unpaired, the endoderm-disc {E8). On the latter 



Fig. 443. — Early embryo of Astacus. BM, mesoderm ; ES, endoderm disc ; K' , head-lobes ; 

 TA, thoracico-abdominal rudiments. (From Lang's Comparative Anatomy, after Reichenbach.) 



an invagination of the blastoderm takes place, giving rise to a 

 small sac, the archenteron, which communicates with the exterior 

 by an aperture, the blastopore. By this process the embryo passes 

 into the gastrula-slage, which, however, differs from the corre- 

 sponding stage in the types previously studied in the immense 

 quantity of food-yolk filling up the space (blastocoele) between 

 ectoderm and endoderm. Very soon the embryo become tri- 

 ploblastic, or three-layered, by the budding off of cells from the 

 endoderm in the neighbourhood of the blastopore : these accumu- 

 late between the ectoderm and endoderm, and constitute the 

 mesoderm. 



Before long the blastopore closes, converting the archenteron 

 into a shut sac (Fig. 445, A) : the thoracico-abdominal rudiments 

 unite with one another, forming a well-marked oval elevation 



