SIPHONAPTERA AND DIPTERA 



369 



about two hours before the edges of the ectoderm grow together and fuse 

 along the dorsal mid-longitudinal line. 



At 56 hours outgrowths that are the first evidence of the developing 

 Malpighian tubules appear at the end of the proctodaeum. They develop 

 into elongate slender tubules that can be seen distinctly in the later 

 stages of the embryo. About 10 hours later the proctodaeum bends 

 sharply in the form of a letter S. This bend soon forms a complete 

 loop. The proctodaeum twists so 

 that it can no longer be cut longi- 

 tudinally in a verticle plane 

 through the center of the egg. At 

 90 hours the loop is completed. 



At the eightieth hour the mes- 

 enteron is still composed of a long 

 shallow trough holding the yolk. 

 Ten hours later the mid-gut is 

 completed. Two gastric coeca 

 form as outgrowths of the mid-gut 

 wall at its anterior end. At this 

 time the esophageal valve is form- 

 ing. It develops from the inner 

 end of the stomodaeum as an 

 invagination into the mid-gut. As 

 the stomodaeum lengthens, the 

 membrane closing its inner end 

 enlarges into a loose sack hanging 

 down into the yolk (Fig. 326, 

 lim. m) . Sections cut at successive 

 stages after the stomodaeum pushes 

 into the mid-gut show that it 

 greatly increases in length and 

 resembles as to position the peri- 

 trophic • membrane of the larva. 



suboesg 



stom.p 



mge 



lim.m 



proct 



Fig. 326. — Sciara. Longitudinal section 

 of 85-hour embryo, {ect) Ectoderm, {ggl) 

 Ganglion, {lim. m) Limiting membrane. 

 {mge) Mid-gut epithelium, {jproct) Procto- 

 daeum. {ser) Serosa, {stom) Anterior (a) 

 and posterior (p) ends of stomodaeum. 

 {suboesg) Subesophageal ganglion. 



Studies have been made indicating a 

 definite relationship between the stomodaeal membrane and the peri- 

 trophic membrane (Gambrell, 1933). 



Longitudinal sections taken just after the beginning of segmentation 

 and the invagination of the neural ridge show masses of mesodermal cells 

 lying in the segmentation cavities on both sides of the neural ridge. 

 These are more than one layer thick in contrast to the single inner layer 

 from which they were derived. 



Shortly after the fiftieth hour a constriction appears in the mesoderm 

 that divides it into two parts, an ental and an ectal part, as shown in 

 Fig. 324 at the 60-hour stage. The ental part lying next to the mesen- 



