404 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



The Alimentary Canal. — The origin of the entodermal cells has 

 already been mentioned. They form a thin layer on the ventral side of 

 the egg immediately outside the yolk (Figs. 340^, 346, ent). This layer 

 spreads, apparently by a process of stretching, over the entire yolk as 

 a thin-walled sac before the formation of the individual coelomic sacs. 

 The yolk cells (trophocytes, or vitellophags) are similar to entoderm cells 

 in physiological and morphological characters, the nuclei being usually 

 larger, often irregular in shape, with one or more nucleoli. The yolk 



*%®ais — br 



Fig. 360. — Scolopendra cingulata. Sagittal section of the stomodaeum. {br) Brain. 

 ifg) Anlage of frontal ganglion. (Zr) Labrum. {mus) Muscle, ijn) Recurrent nerve. 

 {sin) Sinus. {Adapted from Heymons.) 



cells in the egg center break down by the time segmentation of the germ 

 band is completed, and soon thereafter the yolk pyramids also dis- 

 integrate, the yolk being reduced to a homogeneous mass by the time the 

 lateral halves of the germ band separate from each other (Fig. 341^4). 

 By this time or soon afterward the yolk cells become more uniformly dis- 

 tributed through the yolk (Fig. 348) and are then characterized by their 

 much larger nuclei, preliminary to their degeneration. The entodermal 

 cells, which now form a sac-Uke layer around the yolk, take over the 

 function of reducing the yolk. Meanwhile the stomodaeum and proc- 

 todaeum have formed as simple sac-like invaginations (Fig. 360), the 

 former arising shortly before the latter. The dorsal wall of the stomo- 



