THE ALIMENTARY CANAL 85 



In Carausius morosus, according to Leuzinger (1926), yolk cells are 

 set free from the median line of the ental surface of the embryonic rudi- 

 ment, cells that connect with each other by pseudopod-like processes to 

 form a reticulated sheet between the rudiment and the yolk. This sheet 

 is the yolk-cell membrane, or lamella, and is interpreted b.y Leuzinger as 

 the primary entoderm. From the mesal margins of the segmented inner 

 layer mesodermal cells are liberated which pass by way of pseudopod-like 

 plasma bridges into the yolk cell membrane along the median longitudinal 

 line, a position from which they migrate laterally, and replace the original 

 yolk cells of the membrane, crowding them into the yolk where they 

 undergo degeneration. Although the replacement cells are regarded by 

 Leuzinger as mesodermic, the membrane itself, after substitution, is 

 designated by him as the secondary entoderm. Thomas (1936) states 

 that the definitive digestive epithelium originates from mesenteron rudi- 

 ments located at the tips of the stomodaeal and proctodaeal invaginations 

 in Carausius, but both Hammerschmidt (1910) and Leuzinger and Wies- 

 mann (1926) expressly deny this. 



Nelsen (1934) states that in Melanoplus differentialis the mid-gut 

 rudiment is derived from the inner layer associated with the blind ends 

 of the stomodaeum and proctodaeum, although he does not deny the 

 possibility of ectodermal material migrating inward with the entoderm. 

 The mid-gut epithelium thus develops from anterior and posterior 

 rudiments. 



Stuart (1935), in his study of this species, arrived at a different con- 

 clusion regarding the origin of the mid-gut. He maintains that shortly 

 before hatching of the nymph the yolk cells move peripherally to form a 

 temporary lining upon the inner surface of the mesodermic components 

 of the mid-gut. About the ' time the insect hatches, each yolk-cell 

 nucleus divides into a dozen or more smaller nuclei, which Stuart desig- 

 nates as the presumptive mid-gut epithelial nuclei. Each appropriates 

 a portion of the yolk-cell cytoplasm and then becomes a definitive mid- 

 gut epithelial cell. In Forficula the mid-gut epithelium is formed from 

 a median dorsal ribbon with forked end growing out from the stomod- 

 aeum and another one from the proctodaeum. They are called "ecto- 

 dermic" by Heymons (1895), but Strindberg (1915) maintains that the 

 mid-gut ribbons in Forficula arise from entodermic rudiments. 



NEUROPTEROIDS 



Strindberg (1913) states that in the termite Eutermes rotundiceps the 

 inner layer is composed of an outer sheet of mesoderm and a thin inner 

 sheet of entoderm which are readily distinguished from each other by 

 their nuclear structure. After the formation of the coelomic sacs both 

 mesoderm and entoderm are interrupted along the median line. Imme- 



