TRICHOPTERA AND LEPIDOPTERA 



337 



mx2 



outer, or somatic, mesoderm becomes closely united to the ectoderm, 

 extending dorsally to form the outer wall of the sides and the dorsum. 

 Before the folds meet on the middorsal line, the amnion and serosa are 

 ruptured. 



The yolk or entoderm cells increase very rapidly both in number 

 and in size before the dorsal closure. 

 Before the splanchnic mesoderm has 

 completely separated the yolk from 

 the body cavity on the ventral side, 

 some of the yolk cells migrate into 

 the body cavity (Fig. 291,i/c) where 

 they arrange themselves irregularly 

 along the sides of the body wall. At 

 first the cells are arranged singly and 

 indefinitely along the outer wall of 

 the body (Fig. 292), but later, by 

 increasing in numbers, they become 

 arranged in distinct groups. These 

 entoderm (yolk) cells are both more 

 distinct and more numerous in the 

 posterior part of the yolk sac, the 

 epithelial lining of the stomach prob- 

 ably being formed first in the neighborhood of the proctodaeum and then 

 extending forward to the stomodaeum. It is certain that the yolk cells 

 (entoderm) do not form a continuous sac until some time after the 

 formation of the mesodermic musculature of the yolk sac 



Fig. 293. — Neophylax. Lateral view 

 of right side after revolution, (md) 

 Mandible, (mx 2) Labium, (p 1) First 

 thoracic leg. 



.K4^ 





yc 

 Fig. 294. — Neophylax. Cross section of advanced embryo. (6. cv) Body cavity, {h) 

 Heart, {som. m) Somatic mesoderm, (splm) Splanchnic mesoderm, {yc) Yolk cells. 



The last stage in the development of the embryo starts when the 

 embryonic envelopes rupture. The process of the elimination of the 

 envelopes is the same as wdth the dragonflies, the earwigs, and other 

 insects, a secondary dorsal organ being produced which later sinks into 

 the yolk and is absorbed. After the rupture of the membranes and dur- 

 ing the changes that take place in the dorsal organ, the abdomen of the 



