188 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



differs from that of most insects. When the serosa is completed, the yolk 

 will be seen to have divided into numerous, not very sharply defined yolk 

 spherules, or compartments, within each of which is a yolk nucleus (Fig. 

 109). The yolk nuclei, derived from yolk cells, differ but little from 

 those of the germ disk. After the young larva leaves the egg, the yolk 

 spherules separate from each other, retracting toward the mesodermal 

 wall of the mid-gut, thus leaving a lumen which is filled with a fluid, 



Fig. 111. — Lepisma. (am) Amnion. (Ir) Labrum. (,md) Mandible, (mx) Maxilla 

 iproct) Proctodaeum. (ser) Serosa. 



probably in part derived from dissolved albuminous and fatty substances. 

 After the rupture of the stomodaeal membrane, the lumen extends into 

 the fore-gut. Some of the yolk nuclei, each of which is now surrounded 

 by a plasma layer and therefore properly called a "yolk cell," migrate out 

 of the yolk spherules. Perhaps instead of a migration it may rather be a 

 dissolving and absorption of the yolk substance. However that may be, 

 the yolk cells attach themselves to the mid-gut mesoderm wall, where they 

 undergo active mitotic division, thus forming here small clusters of five 

 or six cells each (Fig. 112, crypt). Heymons (1897) did not follow the 



