THE EMBRYOLOGY OF THE HONEY BEE 71 



spherical cell masses of which the posterior is the larger, their 

 convex surfaces directed inward. Later the superficial layer 

 of these rudiments is modified to form ectoderm, with the ex- 

 ception of a small area situated over the center of the anterior 

 rudiment, which remains unmodified and actively proliferating, 

 and which is to form the floor of the stomodaeal invagination. 

 This area corresponds to a similar area over the anterior 

 mesenteron rudiment of the honey bee, described above. Car- 

 riere and Burger, in opposition to Grassi, insist strongly on the 

 independence of the mesenteron rudiments from the mesoderm, 

 and consider those parts of the ventral plate from which the 

 mesenteron rudiments take their rise as purely blastodermal. 



Only one other paper in the social Hymenoptera remains to be 

 mentioned in this connection, that of Dickel (1904) on the honey 

 bee. This is unique in that it seeks the origin of the "entoderm" 

 in a peculiar discoid cell mass appearing at the anterior end of 

 the egg during the earlier stages in the formation of the germ 

 layers, and derived from yolk cells, and therefore turned "yolk 

 plug" or "yolk syncitium." A corresponding "yolk plug" was 

 assumed by Dickel to exist at the posterior end of the egg. 

 These cell masses are supposed to be later carried inward by 

 invagination and to constitute the anterior and posterior mesen- 

 teron rudiments. In a succeeding section the origin and fate of 

 the "yolk plug" of Dickel will be discussed in detail ; it is suffi- 

 cient to state here that it has no connection with the mesenteron 

 rudiments. The invagination figured by Dickel at the anterior 

 end of the egg can readily be construed as an artifact, since such 

 infoldings are very common in eggs of the bee which have not 

 been properly handled and are frequently produced by the os- 

 motic pressure of a clearing agent, such as cedar oil, when 

 incautiously used. 



The origin of the mesenteron of insects has for the past forty 

 years been the subject of numerous investigations and also a 

 prolific source of discussion, from which the partisan spirit has 

 not been altogether absent. In the history of embryological re- 

 search there is perhaps no problem about which there has been a 

 greater diversity of opinion, and it is a regrettable fact that even 

 at the present day investigators of this subject are still arrayed 

 against one another in opposing camps. This is still more regret- 





