THE CRAYFISH 



141 



exterior by a wide mouth or blastopore. The pocket is the 

 mid-gut or mesenteron, and the cells forming its walls are hypo- 

 blast. During, and even before the invagination, the cells of 

 the primitive streak lying in front of the blastopore divide 

 rapidly, and give rise to a small mass of cells lying between 



Ji. 



21. 



A. A recently fertilised ovum of Astacus showing the central 

 nucleus , in the act of dividing. B. A later stage in which 

 the nuclei form a peripheral layer enclosing the yolk. C. The 

 ovum is divided into a number of yolk-pyramids, jy/- D. The 

 inner ends of the yolk -pyramids have fused to form a central 

 mass of yolk enclosed by bd, the blastoderm, composed of 

 flattened cells, st, the stalk by which the egg is attached to 

 the limbs of the parent. (A,.B and D after Moren, C after 

 Reichenbach.) 



hypoblast and blastoderm in front of the blastopore. This 

 mass of cells is the mesoblast. The invagination of the 

 mesenteric cavity progresses some little way till a flattened sac 

 is formed lying between the yolk and the epiblast; then the 



