74 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



About the time of hatching, each vitellophag nucleus divides in a dozen 

 or more smaller nuclei, which are designated by Stuart as "presumptive 

 mid-gut epithehal nuclei," each of which appropriates a portion of the 

 vitellophag cytoplasm and thus forms the definitive mid-gut epithelium 

 cells. At times the vitellophag nuclei may divide while still out in the 

 lumen of the mesenteron. 



The junior author, in a still unpublished account of the development 

 of dragonflies (Plathemis, Erythemis, Libellula), agrees with Tschuproff's 

 statement that the vitellophags take part in the formation of the middle 

 section of the mid-gut epithelium. This is discussed in Chap. XIV. 



Attention should be called at this point to the fact that peripheral 

 migration of yolk cells has been reported by a number of investigators for 

 Isoptera, Orthoptera, Plecoptera, and Hymenoptera. Miller (1939) has 

 described a nucleated membrane, presumably derived from yolk cells, 

 which lies in the lumen of the mid-gut and which is shed after hatching 

 of the nymph. The definitive mid-gut epithelium here has apparently 

 been formed by proliferation of cells from the blind end of the procto- 

 daeum. A somewhat similar case is reported by Hoffman (1913-1914) 

 of the development of species of Strepsiptera, wherein the primary mid- 

 gut arises from three vitellophags which surround a lumen. By cell 

 proliferation from the bhnd ends of the stomodaeum a layer of ectodermal 

 cells grow over the primary mid-gut to form the epithelium of the second- 

 ary mid-gut. The primary mid-gut is now absorbed, leaving, according 

 to Hoffmann, a definitive epithelium composed of ectoderm only. The 

 three yolk cells are comparable to the annelid macromeres both in their 

 mode of origin by total cleavage and in their fate. Noskiewicz and 

 Poluszynski (1928) have come to a similar conclusion for Stylops. The 

 formation of the yolk-cell membrane in Carausius (Leuzinger and Wies- 

 mann, 1926) is also analogous. The membranous cellular sacs that are 

 formed within the lumen of the mesenteron of a psocid described by 

 Fernando may also belong to this category (see Chap. XVI). For lack of 

 sufficient data a positive statement as to the part played by the primary 

 yolk cells in connection with the formation of the mid-gut epithelium in 

 the Apterygota, the Odonata, certain Orthoptera, and other Pterygota 

 cannot be made at the present time. In view, however, of the works 

 cited above, the subject cannot be lightly dismissed. Renewed investiga- 

 tion of the development of yolk cells in apteryotes and in some of the 

 primitive pterygotes is necessary to solve this problem. 



Whatever may be the derivation of the mid-gut epithelium it is 

 probable that the majority of embryologists are now agreed that the 

 epithelium develops from the same germ layer in most pterygotes. 

 Roonwal (1937) and some others, however, believe that this tissue may 

 originate from ectoderm or secondary entoderm or both. 



