42 THE EMBRYOLOGY OF THE HONEY BEE 



stricted base. There is however, this difference : that in Hydro- 

 philus the blastoderm cells maintain continuously their connec- 

 tion with the inner cortical layer, and are never separated from 

 it by a basement membrane. In Musca, however, in certain parts 

 of the egg, the inner cortical layer is separated from the blasto- 

 derm cells by a layer of yolk granules. By the inward (centrad) 

 extension of the blastoderm cells, this layer of yolk, as well as 

 the inner cortical layer, is taken up into the bases of the cells. 

 This process seems to take place much more slowly and at a 

 later period than in Hydrophilus. In this respect, as well as in 

 the separation of the inner cortical layer from the blastoderm 

 cells, Musca may be said to approach the honey bee more closely 

 than Hydrophilus. 



Another feature of interest is the long duration of the blasto- 

 dermal period — 16 to 18 hours, approximately, or nearly one 

 quarter of the total length of time required for complete de- 

 velopment in the egg. In Hydrophilus (Heider 1889), the only 

 insect concerning which full and accurate data are given of the 

 duration of each stage, the period required for the formation 

 and completion of the blastoderm (from the close of cleavage to 

 the first appearance of the lateral plates) is about 27 hours. The 

 total time required for development in the egg is 11 days, or 

 274 hours. The blastodermal period then occupies a trifle more 

 than one-tenth of the entire time, as compared with one-fourth 

 in the honey bee. This comparison, however, is perhaps a little 

 unfair, since the larva of Hydrophilus at hatching is fitted for 

 an active existence and probably therefore more highly differ- 

 entiated than the larva of the bee at a corresponding stage ; 

 nevertheless, the comparison is a striking one. 



The significance of the long sojourn of the bee's egg in the 

 blastodermal period can only be guessed at ; its importance can 

 only be estimated by its duration. Compared with the changes 

 occurring within the egg in any other equal space of time the 

 morphological changes during this period are insignificant. This 

 leads naturally to the surmise that the nature of the changes 

 undergone by the egg at this time may be principally physiologi- 

 cal, and therefore not made evident by the ordinary methods of 

 the embryologist. 



