THE EMBRYOLOGY OF THE HONEY BEE 255 



their inner ends becoming sharply defined. The remnant of the 

 inner cortical layer clings to the yolk over which it now forms a 

 pellicle, this being separated from the blastoderm by a narrow 

 space. The blastoderm is at first of nearly uniform thickness 

 around the circumference of the egg, but soon becomes differen- 

 tiated into a thicker ventral and a thinner dorsal portion. Along 

 the dorsal mid-line is a strip of cells differing from the remainder 

 in being especially thin and flat and in maintaining a close relation 

 with the yolk. 



The mesoderm is formed from a median area of the ventral 

 blastoderm, the middle plate, which separates from the blasto- 

 derm on each side of it and which constitutes the lateral plates. 

 The ventral plate is then covered over by the lateral plates, which 

 approach one another, and finally become united along the ventral 

 mid-line to form the ectoderm. The rudiments of the mesenteron 

 are formed by the immigration of blastoderm cells, a discoid 

 swelling being thus produced at each of the two ends of the 

 middle plate, but outside of its limits. These rudiments later 

 become covered by ectoderm. The anterior mesenteron rudiment 

 does not however become completely covered, a small circular 

 area near its caudal margin remaining uncovered and later con- 

 stituting the floor of the stomodaeal invagination. During these 

 developmental changes the median dorsal area of the blastoderm, 

 composed of thin flat cells, also becomes depressed and is over- 

 grown by the dorsal margins of the lateral plates. Meanwhile 

 the cells of this dorsal strip have becme aggregated in the cephalo- 

 dorsal region of the egg to form a more or less discoid mass, the 

 cephalo-dorsal body, the so-called "yolk plug" of O. Dickel 

 (1904). During the formation of the so-called "germ-layers" 

 both the middle and lateral plates show plain evidence of segmen- 

 tation. The segments thus indicated appear to correspond to the 

 definite segments of the embryo. 



The amnion is single layered and is formed from the dorsal 

 half of the blastoderm. This separates both from the yolk and 

 from the margins of the ventral or embryonic half of the blas- 

 toderm, the ventral plate. This separation does not take place 

 simultaneously, but occurs first at the cephalic end of the egg, a 

 cap-like fold being formed which grows rapidly caudad. A sim- 



