MYRIAPODA 



405 



daeum (Fig. 360) will give rise to the stomatogastric nervous system, 

 hence is thicker than the ventral wall. The mesoderm (mus) of the 

 stomodaeum arises in large part from cells that were present in the median 

 line in the region of the mouth, that of the proctodaeum from the 

 mesoderm cells of the telson. The coelomic sacs apparently do not con- 

 tribute to the formation of muscles of either stomodaeum or proctodaeum. 

 The ectodermal cellular layer at the blind ends of both invaginations by 

 this time has become very thin (Fig. 361, Im). Two diverticulae are formed 

 at the blind end of the proctodaeum which represent the anlagen of the 

 Malpighian tubules. Although the entoderm (Fig. 361) elsewhere forms 

 a layer of flattened cells, in the periphery of the blind end of the proc- 

 todaeum they are more or less columnar. In the first embryonic stage, 



Fig. 361. — Scolopendra cingulata. Sagittal section of proctodaeum. (bl) Blood cell, 

 (enf) Entoderm, (/m) Limiting membrane of proctodaeum. (mus) Muscle layer, (tel) 

 Telson. (Adapted from Heymons.) 



before the first molt, both stomodaeum and proctodaeum as well as the 

 two Malpighian tubules become elongated. At this time also the 

 entodermal cells begin to feed on the adjacent yolk, thereby changing in 

 shape from flattened cells into deep, vacuolated cells (Figs. 357, 358, ent) 

 in which fat and yolk droplets may be detected. The absorption of 

 yolk is apparently correlated with the rapid growth of the embryo at 

 this stage. After the first molt the yolk cells still remaining in the yolk 

 begin to disappear. The rupture of the blind ends of stomodaeum and 

 proctodaeum and the formation of an intima take place during the post- 

 embryonic period. During this period also the sac-hke mid-gut becomes 

 materially constricted, a condition brought about by the elimination of 

 numerous cells of the entodermal epithelium into the lumen of the gut. 

 As outlined above, the first developmental period extends from the 

 time of cleavage to the formation of the germ layers. The second 

 includes the time of segmentation until the first appearance of the rudi- 



