GASTRULATION, FORMATION OF GERM LAYERS 



71 



the lips of the blastopore (Fig. 42). The cellular layer derived from the 

 gastrular invagination (the inner layer) according to Kowalewsky's theory 

 represents the anlage of the entoderm and mesoderm; the former (Fig. 

 43 A, mge), having been pulled apart, is restricted to an anterior and 

 posterior entoderm rudiment, the latter constituting the major portion of 

 the inner layer. Some recent writers (Hirschler, Fulinski) hold that in 

 some embryos the two entoderm rudiments are connected by a median 



mge. a 



mge.p 



A B 



Fig. 43. — Sagittal section of embryo. Development of the mid-gut epithelium, (am) 

 Amnion, {ect) Ectoderm, (mes) Mesoderm, (mge) Mid-gut epithelial rudiment: (a) 

 anterior, (p) posterior, (proct) Proctodaeum. (rb) Mid-gut epithelial ribbon, {stom) 

 Stomodaeum. 



strand, coming back to Rabl's idea of the inner layer being composed of a 

 median entoderm band and paired mesoderm bands. Kowalewsky has 

 compared the formation of the germ bands in insects with their formation 

 in Sagitta where the archenteron becomes divided by the appearance of 

 two folds into a median enteric rudiment and two lateral coelomic sacs 

 (Fig. 40). The views of Grassi and Kowalewsky were later accepted by 

 Wheeler, Cholodkowsky, and others. 



A number of writers, on the other hand, following Heymons (1895o) 

 maintained that the mid-gut epithelial rudiments arose from the blind 

 ends of the stomodaeum and proctodaeum in the Orthoptera and some 

 other orders. They came to the conclusion that the functional mesen- 

 teron of pterygote insects is of recent origin and that the original entoderm 



