240 THE EMBRYOLOGY OF THE HONEY BEE 



but coarse meshes. Soon it becomes ragged, irregular and finely 

 granular, ultimately, at Stage XV acquiring the appearance of a 

 pale and finely granular precipitate. 



The yolk cells have been given but slight consideration by most 

 of the investigators of insect embryology. The reason for this 

 is not far to seek. Many of the earlier embryologists looked 

 upon the yolk cells as the material from which the mesenteron 

 was formed, and therefore corresponding not only theoretically 

 but actually to the entoderm of other animal forms. When, how- 

 ever, it was found that the mesenteron in the pterygote insects 

 was derived from other sources, interest in the cells of the yolk 

 waned, and consequently published observations on them are, so 

 to speak, sporadic and scattered. 



The origin of yolk cells from cleavage cells which do not 

 migrate to the periphery of the yolk, but remain behind, was es- 

 tablished by Bobretzsky (1878) in the case of the butterfly Pieris. 

 This mode of origin of the primary yolk cells was confirmed by 

 many other investigators. Grassi's observations on the honey 

 bee (1884) seem to be entitled to a place among these, although 

 his statements on this point are not perfectly clear. The origin 

 of yolk cells in this manner has subsequently proved to be the 

 rule among pterygote insects, although exceptions were early 

 noted. Thus Patten (1884) in Neophylax and Korotneff (1885) 

 in Gryllotalpa, found that all of the cleavage cells migrate to the 

 surface of the egg. Later some of these wander back into the 

 yolk to form the primary yolk cells. This is commonly taken 

 to be a primitive condition, based on the assumption that the 

 primary yolk cells represent the entoderm. This retrograde 

 migration must then be regarded as a modified process of gas- 

 trulation like that found in arachnids. 



A second characteristic which the yolk cells of the bee possess 

 in common with certain other insects is the large size of these 

 cells. It is certain that this is true of the Dermaptera and Orthop- 

 tera, since Heymons (1895) calls especial attention to it and it is 

 also true, according to Lecaillon (1898) for the chrysomelid 

 beetles Chrysomela, Lina and A g elastic a, but not for Clytra. It 

 seems doubtful how far this relation obtains among insects in 

 general. Definite statements on this point are wanting and an 



