516 



ZOOLOGY 



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. 409, K), on which the eyes subsequently appear ; two some- 

 what further back, the thoracico-abdominal rudiments (TA)', and 

 one, posterior and unpaired, the endoderm-disc (J2S). On the latter 

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

 small sac, the archenteron, which communicates with the exterior 

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



Fio. 409. Early embryo of Astacus. BM mesoderm ; ES, endoderm disc ; K', head-lobes 



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



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

 sponding stage in the types previously studied in the immense 

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

 ectoderm and endoderm. Very soon the embryo becomes 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. 411, A) : the thoracico-abdominal rudiments 

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

 (Fig. 410, TA), and three pairs of elevations appear between i 

 and the head-lobes. These are the rudiments of the first thr 

 pairs of appendages, the' antennules (a r ), antennae (a. 2 ,), an< ^ 

 dibles (m.)\ by their appearance the embryo passes into th 



