THE EMBRYOLOGY OF THE HONEY BEE 93 



surface into the yolk and becomes covered by blastoderm (am- 

 nion). So far its history corresponds to that of the cephalo- 

 dorsal body. Next the "Rz-ce\ls" glide caudad along the dorsal 

 side of the yolk until they reach the middle of the abdomen, in- 

 creasing in number meanwhile by mitotic division. Here they 

 are overtaken by the advancing ectoderm and the epithelium of 

 the mesenteron and come to lie between these two layers. Finally 

 the "ife-cells" divide into two groups, one on each side of the 

 embryo. These then migrate ventrad, enter the coelomic cavities 

 on each side of the third, fourth and fifth abdominal segments 

 and constitute the male sex cells. 



Nachtsheim has already (191 3) explicitly affirmed the identity 

 of the "ife-cells" of Petrunkewitsch and the "yolk plug," of 

 Dickel solely on the evidence afforded by the two papers. 



The writer has not so far studied the "Rz" cells in the drone 

 egg, so that a final conclusion regarding the identity of the 

 cephalo-dorsal body and the "Rz" cells may seem premature. 

 Nevertheless, a comparison of Petrunkewitsch's figure with the 

 corresponding stages in the worker egg, as well as with the figures 

 given by Dickel (1904) leads one almost inevitably to the con- 

 clusion that the "Rz" cells, the "yolk plug," and the "cephalo- 

 dorsal" body are identical. 



The significance and possible homologies of the cephalo-dorsal 

 body are extremely uncertain. Its rapid disappearance and great 

 variability suggest that it is a vestigial structure. Hirschler 

 (1909) has described, in the chrysomelid beetle Donacia, a struc- 

 ture termed by him "the primary dorsal organ," which in several 

 respects resembles the cephalo-dorsal body of the honey bee. This 

 "primary dorsal organ" consists of an oval area of the blastoderm 

 lying in the dorsal mid-line, which sinks down into the yolk and 

 becomes covered over by blastoderm, its component cells mean- 

 while sending out processes into the yolk and also showing evi- 

 dences of degeneration. Finally "the primary dorsal organ" de- 

 generates completely and is absorbed by the yolk. In position, 

 overgrowth by blastoderm and final absorption by the yolk this 

 structure corresponds quite closely to the cephalo-dorsal body of 

 the honey bee, but in one respect it differs : in the bee the cephalo- 

 dorsal body is formed during the formation of the germ layers, 



