SIPHONAPTERA AND DIPTERA 



367 



At the fiftieth hour the cells lining the neural groove may be seen in 

 cross section as two parallel vertical rows of neuroblasts formed in the 

 neural ridge. They soon are modified as the ridge widens out and show 

 the presence of ganglia before the sixty-fifth hour. The ganglia become 

 distinct with the appearance of the neuropile. The ectoderm separates 

 from the nerve cord at the same time. The neuropile of the brain 

 appears before that of the ganglia of the nerve cord, and at the eightieth 

 hour the circumesophageal commissure is distinct. 



At the time segmentation begins, the stomodaeum appears as a 

 shallow depression in the ectoderm in the anterior region of the embryo. 

 At first this depression is no deeper than those formed by the process of 

 segmentation, but it soon becomes a deep tube closed at its inner end. 



•neurg 



Fig. 323. — Sciara. Cross section of 50-hour embryo, {mes) Mesoderm, (mge) Mid-gut 

 epithelial ribbons, (neurg) Neural groove. 



In the meantime the inner layer, during the process of segmentation, 

 divides into transverse masses of cells that lie in close contact with the 

 segmentally divided ectoderm. One of these cell masses of the inner 

 layer, no different in appearance from the others, lies around the base of 

 the stomodaeum. It corresponds to the anterior mesenteron rudiment 

 in the muscoidean Diptera but is not pushed into the yolk on the end of 

 the invaginating stomodaeum. 



The cells of the anterior mesenteron rudiment, multiplying rapidly, 

 grow over the end of the stomodaeum and send out posteriorly two slender 

 ribbons into the yolk. These are the branches of the anterior mesenteron 

 rudiment that will form the anterior end of the mid-gut. These branches 

 develop from the ventral side of the mesenteron rudiment and are only a 

 few cells in width. They lie at the sides and above the neural ridge, in 

 close contact with the mesoderm (Figs. 323, 324, mge). 



The proctodaeum is not discernible until nearly the fiftieth hour, 

 when the posterior end of the embryo loses practically all its curl by 

 shortening and lies again on the dorsal side. The formation of the pos- 



