THE ALIMENTARY CANAL 83 



Among certain other Coleoptera and Lepidoptera, as well as Orthop- 

 tera, the mid-gut epithelium seems to originate directly from proliferat- 

 ing ectodermal cells that lie in the tip of the blind ends of the stomodaeum 

 and proctodaeum. In this case also, ribbons or a middle strand or both 

 may give rise to the mid-gut epithelium. 



As has been stated in the preceding chapter, it seems probable that 

 from whatever germ layer the mid-gut is assumed to arise, its origin is 

 the same in the two cases cited above. 



Lastly, in the myriapods and apterygotes, as well as with the Odonata 

 and perhaps other primitive pterygotes, the yolk cells are regarded by 

 some embryologists as the source from which at least a part of the mid- 

 gut is derived. In this chapter the terms "entoderm" and "ectoderm" 

 are used as applied by the authors cited. 



MYRIAPODA 



Chilopoda.— The entoderm cells which are to form the mid-gut epi- 

 theUum in Scolope/ndra (Heymons, 1901) are set free from numerous 

 points of the inner ventral surface of the blastoderm and then collect in a 

 thin layer on the ventral side of the yolk. This layer soon spreads, 

 apparently b}^ a process of stretching, over the entire yolk as a thin- 

 walled sac, thus forming the epithelium. 



Diplopoda. — Pflugf elder (1932) found that the entoderm in Platy- 

 rhacus arises by active proliferation of cells at a point where the future 

 stomodaeum will form. Cells liberated here migrate toward the posterior 

 pole ; the cells at the seat of proliferation become fewer, leaving an open- 

 ing at the surface that projects itself into the proliferating cell mass to 

 form a lumen. Later the stomodaeum and the proctodaeum are estab- 

 lished, while the entodermal cells arrange themselves epithelial fashion 

 forming the mid-gut. In Julus, Heathcote (1886) found that the mid-gut 

 epithelium is derived from yolk cells, some of which collect in the median 

 line just entad of the mesoderm where they arrange themselves between 

 stomodaeum and proctodaeum. 



Several workers are in agreement that in the diplopods the mid-gut 

 epithelium arises from a loosely formed strand of cells which is solid at 

 first but later acquires a lumen, the strand extending through (not sur- 

 rounding) the yolk, a most peculiar mode of development. 



COLLEMBOLA 



The mid-gut epithelium in Isotoma cinerea, according to Philip- 

 tschenko (1912), arises from an anterior and a posterior mesenteron rudi- 

 ment and a middle strand. The two rudiments each give rise to a pair 

 of entoderm ribbons ; the cells liberated from the middle strand migrate 

 to the yolk surface where they later unite with the paired ribbons which 



