516 



ZOOLOGY 



SECT. 



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 tlioradco-aMominal rudiments (TA); and 

 one, posterior and- unpaired, the endoderm-disc (JUS). 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 blastopore. By this process the embryo passes 



FIG. 409. Early embryo of Astacus. BM mesoderm ; ES, endoderm disc; A"', head-lobes ; 

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



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 (blastocoele) 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 it 

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

 pairs of appendages, the antennules ( r ), antenna^ (.,.), and man- 

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



