166 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The inner layer is formed by a division of the cells of the blastoderm, 

 giving rise finally to a complete second layer of cells below the first 

 (Fig. 68). Within are the yolk cells (yc), differing from the peripheral 



Fig. 64. — Isotoma. Egg in eight-cell 

 stage. 



Fig. 65. — Isotoma. 64-cell stage. 



cells in being vacuolate, and the cluster of germ cells {gc) now avcII 

 differentiated. 



The embryo has now become oval with a somewhat accentuated pole 

 where the dorsal organ will develop. The cells of this organ (rfc), devel- 

 oped from the outer layer and increasing in depth, soon are sharply 



Fig. 66. — Isotoma. Peripheral migration of cleavage cells. 



differentiated from the adjacent cells, beneath which the inner layer of 

 cells degenerate (Fig. 69). The cells of the dorsal organ later become 

 elongate and strongly vacuolate. While this organ is developing, the 

 outer layer of cells of the embryo begins to secrete a cuticula. After 



