SIPHON APTERA AND DIPT ERA 



371 



when the cleavage nuclei enter this layer. It then becomes thicker when 

 the nuclei reach the surface. The closing in of the blastoderm under the 



JO ^ 



Fig. 327. — CalUphora erythrocephala. A-C, Successive stages. D, longitudinal 

 section, {bid) Blastoderm, (cc) Cleavage cell nucleus, (gc) Germ cells, (os) Oosome. 

 (pr) Periplasm, (pr. i) Inner periplasm, {y) Yolk, (j/c) Yolk cell. 



germ cells is not complete, but there is formed here a funnel-shaped 

 invagination, from the margin of which a migration of nuclei begins into 

 the egg mass where they become yolk cells (Fig. 327C). 



