THE GASTRULA INVAG1NATION 



The cellular layer arising from the gastrula invagination (lower 

 layer) forms the common germ of the endoderm and mesoderm. It 

 has only recently become known how these two germ-layers of insects 

 have become differentiated. Kowalevsky first discovered in Musca 

 that the greatest part of the lower (inner) layer yielded mesoderm 

 exclusively, and that a cell-mass only corresponding to the most 

 anterior and posterior end of the primitive band was used in the 

 formation of the endoderm. We must therefore, in insects, speak 

 of a fore and a hinder endodermal rudiment. In proportion, now, 

 as the ectodermal invaginations, which are destined to form the 

 stomodseum and the proctodaeum sink beneath the surface of the 



A c 



FIG. 538. Diagrammatic sketch of the formation of the germinal layers in Doryphora : A, view 

 of upper surface. , cross- section through the fore end of the primitive streak at the line a-a. C, 

 section through the middle of the primitive streak corresponding to the line b-b. D, section through 

 the hinder end of the primitive band corresponding to the line c-c : bl, blastopore ; ec, ectoderm ; 

 en', anterior U-shaped; en", hinder U-shaped germ of the endoderm; ms, mesoderm. After 

 Wheeler, from Korschelt and Heider. 



embryo, the cell-masses of which the two endodermal rudiments are 

 composed are pushed farther in, and a separation between them 

 and the mesoderm is thus effected. The two endodermal rudiments 

 now form accumulations of cells which lie closely adjacent to the 

 blind ends of the stomodeal and the proctodeal invaginations. 

 They soon widen out into two hour-glass-shaped rudiments, which 

 are directed with their concavities towards each other, but with their 

 convex side towards the nearest pole of the egg. They soon change 

 their form ; two lateral stripes grow out from them, and each now 

 assumes the form of a U (Fig. 538, en'). The limbs of the fore 

 2o 



