THE EMBRYOLOGY OF THE HONEY BEE 75 



Friederichs (1906) Donacia. 



(IV) Mesenteron derived, independent of the mesoderm, 

 from two proliferating areas of the blastoderm, one at each end 

 of the germ band, corresponding to the future location of the 

 stomodaeum and proctodaeum, respectively. 



Carriere and Burger (1897) Chalicodoma, Tenebrio. 

 Noack (1901) Calliphora. 



(V) Mesenteron derived from cells migrating inward from 

 thickenings or islands of the blastoderm. 



Uzel (1897, 1898) Lepisma, Campodea. 



The importance accorded by all investigators of insect embry- 

 ology to the question of the origin of the mesenteron is due of 

 course to its relation to the germ layer theory. According to 

 this theory, which is based on a large number of observations 

 on the development of various animals, the material from 

 which the mesenteron is formed should correspond to entoderm 

 and the efforts of practically all investigators of this prob- 

 lem have been bent toward establishing this homology, and 

 to derive the conditions found in the insect egg from the typical 

 gastrula. This has proved to be an exceedingly difficult task. 

 Thirty years ago Weismann (1882) wrote (p. 81): "It be- 

 comes more and more evident, that nowhere in the entire animal 

 kingdom is the ontogeny so distorted and coenogenetically de- 

 generate, as in the insects, so that scarcely anywhere are the 

 germ layers so difficult to recognize as here." Time has proved 

 the truth of these statements. 



Prior to 1884 nearly all of the investigators of insect embry- 

 ology were divided into two camps ; either they followed Dohrn 

 (1866) in deriving the mesenteron from the cells remaining in 

 the yolk, or followed Kowalevski in deriving the mesenteron 

 from the inner wall (splanchnic layer) of the mesodermic so- 

 mites. Only one exception is to be noted: Ganin (1874) de- 

 scribed the mesenteron as derived from the inner ends of the 

 ectodermal proctodaeal and stomodaeal invaginations. In these 

 three views are contained the germs of all the later theories of 

 the origin of the mesenteron. 



In 1884 Grassi's paper on the development of the honey bee 

 appeared, in which the bipolar origin of the mesenteron from 

 the lower layer (mesoderm) was first demonstrated. Kowal- 



