120 THE DEVELOPMENT OF 



Kegarding the entoderm in the hexapods and its relation 

 to the gastrula, various views have been held. The older 

 authors did not trouble themselves much concerning this 

 question, but usually regarded the germinal area as several 

 cells in depth. Kowalevski ('71) was the first to cut sec- 

 tions of the hexapod embryo and to introduce the germ 

 layer theory into the group of arthropods. InHydrophi- 

 lus, Apis, Phryganids and other forms, Kowalevski noticed 

 the groove on the ventral surface of the embryo, and in sec- 

 tions saw arising from the edges of this groove another 

 layer which in Hydrophilus (I. c. pi. ix, fig. 23) contained 

 a distinct lumen . This was very naturally interpreted as 

 an invagination for the production of the entoderm ; but 

 he also discovered that the mesodermal tissues also arose 

 from the same layer, which led him to regard this band of 

 tissue as different from the entoderm (Darmdrtisenblatt) 

 of vertebrates (p. 58). A little later, Haeckel in his pa- 

 pers on the "Gastraea Theorie " ('75 ) adopted Kowalevski's 

 view, considered this a true gastrulation, and regarded the 

 portion thus invaginated as a true entoderm. Hatschek, 

 studying Bombyx ('77), did not pay much attention to 

 this layer, but (p. 117) describes it as small in amount and 

 limited to the most anterior part of the primitive streak, 

 in front of the segmenting embryo. Graber ('78) also 

 regards the process described by Kowalevski in Hydrophi- 

 lus as a true gastrulation and says that in Musca it is so 

 well developed "dass man wirklich, wie bei einer t}'pischen 

 Gastrula, von einer Doppelphase reden kann." In Pyrrho- 

 coris and Lina the process is different, for besides the cells 

 arising from the primitive groove, the inner embryonal 

 cells, which have marked amoeboid characters and which are 

 the 'Wanderzellen' of the older authors, enter into the for- 

 mation of the mesenteron, which thus has a double origin. 



Bobretzky ('78) thinks that, in the Lepidoptera, while 

 some of the cells migrate to the surface to form the bias- 



