34 THE EMBRYOLOGY OF THE HONEY BEE 



nG); the division plane is always normal to the external 

 surface. 



In many accounts of the insect development some details are 

 given of the manner in which the cleavage cells became trans- 

 formed into blastoderm, but probably the most circumstantial 

 accounts are those of Blochmann (iSS/a) and Noack (1901) 

 for Musca; of Heider (1889) for Hydrophilus, Carriere and 

 Burger (1897) for the mason bee (Chalicodoma) and its para- 

 site, Anthophora, and Marshall and Dernhehl (1905) for Poli- 

 stes. The last named account is by far the richest in details. In 

 Musca and Hydrophilus the process (as described) is essentially 

 the same. The cleavage cells, on arriving at the cortical layer, 

 fuse with it to form a continuous sheet without any cell boun- 

 daries a syncitium, in other words in which the nuclei divide, 

 the spindles of the mitotic figures being parallel to the egg's 

 surface. Cell boundaries now appear, first as furrows of the 

 external surface of the egg, thence extending inward, delimiting 

 the cells on their lateral surfaces. Meanwhile the darker stain- 

 ing bases of the cells form a continuous layer within, constituting 

 the "inner Keimhautblastem." In Hydrophilus this is soon ab- 

 sorbed by the cells of the blastoderm. In Musca this layer is 

 much thicker and, in certain portions of the egg, separated from 

 the blastoderm by a layer of yolk. Moreover it is only slowly 

 absorbed by the blastoderm cells, but is ultimately taken up by 

 them. In Anthophora the process is as just described, except 

 that no "innere Keimhautblastem" is formed. It is also absent 

 in! Chalicodoma, as is the cortical layer (Keimhautblastem). 

 The account of Marshall and Dernhehl allows a much closer 

 comparison with the condition met with in the honey bee. In 

 Polistes the cleavage cells enter into the cortical layer separately, 

 with the nuclei situated at the peripheral margin of the cell body, 

 which has the appearance of trailing after the nucleus like the 

 tail of a comet, remaining meanwhile separate from the cortical 

 layer and distinguished by its darker shade, as in the honey bee. 

 A regular inner cortical layer was not observed, but subsequent 

 to the formation of the blastoderm irregular patches of a rather 

 granular mass were found at the bases of some of the blasto- 

 derm cells. The separation of the cell territories was accomp- 

 lished in the usual manner. There is then almost complete agree- 



