78 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Roonwal's statement that the mid-gut may arise from the ectoderm 

 or from the inner layer (secondary entoderm) or ectoderm plus secondary 

 yolk cells would seem to imply that he, at least in part, supports the 

 views of some experimental embryologists as outlined below by Richards. 



2a. Primary yolk cells nongerm-layer derivatives. Inner layer with two 

 components. Primary entoderm buds off into the yolk from the inner wall 

 of the ventral furrow hut gives rise to no larval structures. 



In this group the blastodermal stage corresponds to the blastula stage 

 of other animals. The primary yolk cells are a secondary feature of the 

 developing insect egg. The ventral furrow here is regarded by Mansour 

 (1927) as corresponding to the invagination of the gastrula. Entodermal 

 cells arise from the wall of the ventral furrow or from thickenings in the 

 blastodermal wall. They are budded off into the yolk but give rise to no 

 larval or imaginal structures. The definitive mid-gut epithelium is 

 derived from the inner ends of the stomodaeum and proctodaeum and is 

 therefore of ectodermal origin. Mansour's findings in Calandra oryzae 

 (1927) and those of Heymons (1895a) in the Dermaptera and Orthoptera 

 are thus in agreement so far as the origin of the mid-gut epithelium is 

 concerned, but their views differ as to the significance of the yolk cells. 



2&. Primary yolk cells nongerm-layer derivatives. Median part (pri- 

 mary entoderm) of inner layer, which is confined to bipolar mesenteron 

 rudiments or to a middle strand or to both, gives rise to the mid-gut epithelium. 



Embryologists holding this opinion, following Kowalewsky, regard the 

 peripheral layer of cells first formed by the cleavage cells that have 

 migrated outward as the blastoderm and corresponds to the blastula 

 stage of other animals. The primary yolk cells are a secondary feature. 

 The furrow that may be formed along the midventral line of the germ 

 band is looked upon as a true gastrula furrow, the formation resulting 

 in a gastrula. Further support of this view may be found in the develop- 

 ment of Peripatus capensis in which Sedgwick (1885) has shown that the 

 blastopore divides into two parts by the closure of the middle section 

 (Fig. 42). The two entrances into the gut thus left are spoken of by him 

 as an embryonic mouth and anus. These are subsequently carried 

 inward by the ingrowing stomodaeum and proctodaeum but are never 

 closed. 



The essential difference between this theory and that expressed in 

 section 16 is chiefly a matter of opinion as to the significance of the 

 yolk cells. The following statements based upon observations apply to 

 both. The idea that the mid-gut epithelium is derived from bipolar 

 entoderm rudiments was first expressed by Grassi for the honeybee in 

 1884 and formulated in Kowalewsky's theory two years later. Nusbaum 

 and Fulinski (1906) maintain that in Phyllodromia, in addition to the 

 bipolar entoderm rudiments, there is a connecting strand of entodermal 



