84 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



have lengthened and widened and finally cover the yolk. Both the 

 mesenteron rudiments and the middle strand also include mesoderm 

 elements. Philiptschenko believes that the earlier writers on Col- 

 lembola were in error as to the development of the mid-gut. Uljanin 

 (1885) apparently mistook the germ cells for the mid-gut rudiments. 

 Uzel (1898) for Macrotoma and Achorutes, Prowazek (1900) for Isotoma, 

 and Claypole (1898) for Anurida derived the entoderm from yolk cells. 

 Philiptschenko doubts the yolk-cell origin for Collembola generally on 

 the basis of his study of Isotoma. 



In Isotoma as in other Collembola the closing membrane of the proc- 

 todaeum widens but does not form Malpighian tubules. 



THYSANURA 



Heymons (1897) derives the mid-gut epithelium oi Lepisma saccharina 

 from yolk cells. The yolk spherules retract toward the mesodermal wall 

 of the mid-gut and leave a lumen. Some of the yolk nuclei, each now 

 surrounded by a plasma layer and therefore properly considered yolk 

 cells, migrate out of the spherules and attach themselves to the mid-gut 

 muscle wall, undergoing active mitosis and thus forming small clusters 

 of cells. From these cell crypts, by further proliferation, the mid-gut 

 epithelium is supposed to arise. Some recent writers on theoretical 

 grounds question the yolk-cell origin of the mid-gut epithelium in this 

 species. In the dipluran Campodea staphylinus, which was studied 

 by Uzel (1898), the entoderm cells are said to originate from the germ 

 band but later migrate into the yolk where they become distributed. 

 Their further history is in doubt. 



ORTHOPTERA AND DERMAPTERA 



In the orthopteroid species Ectohia livida, Blattella (Phyllodromia) 

 germanica, Blatta (Periplaneta) orientalis, Mantis religiosus, Gryllus spp., 

 Gryllotalpa vulgaris, Locusta migratoria, Melanoplus differentialis, Hemi- 

 merus talpoides, as well as the dermapteron Forficula auricularia, the 

 anlagen of the mid-gut epithelium are bipolar in position. Although 

 there is an agreement among writers as to location, Heymons and his 

 followers regard these anlagen as ectodermal, whereas other investigators 

 call them entodermal. 



In Stenobothrus, in addition to paired ribbons and apparently inde- 

 pendent of them, there arise from the still intact inner laj^er, from seg- 

 ment to segment, pointed processes which Graber calls "interpolated 

 mid-gut-epithelium anlagen." They grow laterally and segmentally from 

 the middle strand. Therefore in these insects the mid-gut epithelium 

 arises from the inner layer from more than two points of origin. 



