384 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



ventral surface of the egg which become lodged on the yolk surface. At 

 first they are indistinguishable from the yolk cells. Soon more cells are 

 liberated from the cumulus primitivus and from its vicinity. These 

 represent mesoderm or mesenchyme cells. Yolk cells and entoderm cells 

 probably arise chiefly from the ventral side of the egg. The relatively 

 few cells liberated from the inner dorsal side of the egg are probably 

 mainly or exclusively mesenchyme (mesoderm) cells. 



The germ disk represents the posterior part of the future germ band. 

 The heap of cells (Fig. 339) that at first developed on the inner side of the 



mes 



mes 



mes 



Fig. 340. — Scolopendra cingulata. Cross sections A, B, successive stages, {hi) Blood 

 cell, (dm) Membrana dorsalis. {ent) Entoderm cell, {mes) Mesoderm, {vm) Mem- 

 brana ventralis. {y) Yolk, {yc) Yolk cell. {Adapted from Heymons.) 



germ disk becomes dispersed, migrating forward as two lateral mesoderm 

 bands of cells. Meanwhile the ectodermal layer, except on the median 

 line which is now but one cell layer in thickness, gives rise to additional 

 ectodermal cells by radial cell division and mesodermal cells by tangential 

 division (delamination), the latter not encroaching on the median line 

 (Fig. 340A). The occasional isolated cells found on the median line 

 below the ectoderm are doubtless undifferentiated entoderm and mesen- 

 chyme cells, but the cells that are later found between the two lateral 

 mesoderm bands appear to develop into blood cells. The entoderm 

 cells, in contrast to the mesoderm cells, become somewhat flattened and 

 attach themselves to the yolk surface (Fig. 340B) . A median longitudi- 

 nal furrow or invagination is not formed in the germ band. 



