i 3 4 THE EMBRYOLOGY OF THE HONEY BEE 



the hypodermis in the intrasegmental regions and now constitutes 

 the median portion of each ganglion. This separation of the 

 median cord from the hypodermis takes place by a progressive 

 attenuation of the already narrow outer ends of the cells of the 

 median cord until the external portion of the latter becomes re- 

 duced to thin strands, the hypodermis meanwhile closing in from 

 both sides until its lateral halves meet and unite in the mid-line. 

 The hypodermis along the ventral mid-line therefore owes its 

 origin — in the intrasegmental regions, at least, — to hypodermal 

 cells lying originally laterad of this region, as first noted by 

 Grassi (1884). Similar conditions also exist in many other in- 

 sect embryos, Melolontha (Graber 1890) for example. The 

 median cord, after its separation from the hypodermis moves 

 inward, frequently leaving behind it a temporary median notch 

 or groove on the inner surface of the hypodermis, as shown in 

 Fig. 50A. In this figure also are to be seen the delicate strands 

 of protoplasm connecting the median cord and the hypodermis. 

 This separation does not occur simultaneously throughout the 

 length of each segment, but appears to take place first in its 

 posterior half (Fig. 50E). In the intersegmental regions the 

 median cord remains an integral part of the hypodermis (Fig. 

 50C, (MC) ; the lateral cords here separate from both the median 

 cord and the hypodermis proper and constitute the connectives 

 Con). 



In the lateral cords the number of ganglion cells has greatly 

 increased. In every segment a conical group of these lies along 

 each side of the lateral cord, laterad of the neuroblasts, while a 

 well defined single layer covers them on the ventral side. The 

 lateral cell groups apparently furnish the material for the forma- 

 tion of the lateral nerves, while the ventral layer provides the 

 material of the neurilemma. In the dorsal region of the lateral 

 cords a cleft is apparent, separating the uppermost tier of cells 

 from the remainder. These clefts are, so to speak, the forerun- 

 ners of the nerve fibres, which during the succeeding stages 

 traverse these spaces. Laterad of each ganglionic rudiment the 

 mesoderm (Fig. 50B, Meso) presses close against it in the middle 

 region of each segment, frequently, as at Stage X, making it, 

 difficult to accurately determine the lateral limits of the ganglion 

 cells. 



