A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



11 



surrounded b}^ a cellular wall, the so-called "blastoderm," or primary 

 epithelium, which lies immediately below the vitelhne membrane (Fig. 9). 

 Yolk Cells, or Vitellophags. — After the formation of the primary 

 epithelium (blastoderm) some cells migrate back into the yolk where 

 they increase in size and change in appearance (Fig. 9, yc). These are 

 the yolk cells, or vitellophags, which some embryologists regard as the 

 primary entoderm. In the more highly specialized insects the yolk cells, 



c — ^ — -• 



y fe " 



gc- 



^&'~'^0%^ 



Fig. 9. 



Fig. 7. Fig. 8. 



Fig. 7. — Cleavage stage. FiG. 8.' — Blastema stage. FlG. 9. — Blastoderm formation. 

 (bid) Blastoderm, {cc) Cleavage cells, (gc) Germ cell, (y) Yolk, (yc) Yolk cell. 



instead of reaching the periphery and then reentering the yolk, remain 

 behind in the yolk while the cleavage cells continue their outward 

 migration. 



The Germ Band, or Germ Disk, or Embryonic Rudiment, and the 

 Primary Dorsal Organ. — The primary epithelium (blastoderm) now 

 thickens along the midventral longitudinal line, forming the germ band 

 (Fig. 10). The cells on the dorsal and lateral sides of the blastoderm 

 which do not take part in the formation of the germ band become flat- 

 tened, later to form the serosa (ser), or embryonic envelope. In some 

 insects, as in some of the Lepidoptera, the cells while forming the primary 

 epithelium (blastoderm) differentiate immediately into the ventral 

 columnar cells of the germ band and the pavement cells of the serosa. 



