40 THE EMBRYOLOGY OF THE HONEY BEE 



of the nucleus, in those cells situated on the dorso-lateral faces 

 of the egg the vacuoles are larger and much more prominent. 



Thirty-two to thirty-four hours. (Fig. 18A and B.) This 

 stage is distinguished from the preceding principally by two 

 points of difference: first, the blastoderm cells now form a 

 columnar epithelium, they are no longer joined by their bases 

 but limited at their central ends by a well defined basement mem- 

 brane ; second, the mid-dorsal area has again become reduced to 

 a thin sheet of flattened cells. Its point of juncture at the an- 

 terior end with the thick blastoderm of the ventral side has now 

 become shifted to a point ventrad of the cephalic pole (Fig. 18A). 

 The ventral blastoderm has therefore undergone a further short- 

 ening in the long axis of the egg. Of the inner cortical layer 

 there remains a vestige in the form of a very thin granular layer 

 lying near the inner side of the blastoderm. 



Since this stage closes what may be called the blastodermal 

 period of the development, and brings the account down to the 

 formation of the germ layers — which is in fact inaugurated dur- 

 ing this stage — a brief discussion of some of the phenomena 

 characterising this period is in order. 



Among other features, the behavior of the blastoderm cells 

 toward the inner cortical zone deserves especial mention. To 

 review this process briefly: the cleavage nuclei enter the (outer) 

 cortical layer accompanied by a portion of the cytoplasm belong- 

 ing to the cleavage cells, but leaving another darker portion 

 within to form a basal layer, the inner cortical layer, by which 

 the newly formed blastoderm cells are at first united to each 

 other. This is next cut off by the formation of a basement mem- 

 brane. After repeated divisions the cells elongate, the basement 

 membrane disappears and the inner cortical layer is taken up by 

 the cells (22-26 hours). A new basement membrane is then 

 formed (32-34 hours), when the cells have the arrangement and 

 appearance of ordinary columnar (prismatic) epithelial cells. 

 Turning again to the accounts of the development of Hydro- 

 philus (Heider 1889) and Musca (Noack 1901), similar phe- 

 nomena are found to exist in these forms. In Hydrophilus the 

 inner cortical layer is taken up into the blastoderm cells in pre- 

 cisely the same manner as in the honey bee. Moreover the blasto- 

 derm cells afterwards present the same elongated form and con- 



