THE EMBRYOLOGY OF THE HONEY BEE 197 



At Stage VII a tranverse section through the middle of the 

 trunk shows the mesoderm (Fig. 32, Meso) as a double layer of 

 cells lying close beneath the ectoderm, between the latter and the 

 yolk. This condition is typical of the mesoderm from the gnathal 

 region at least as far as the end of the eleventh trunk segment. 

 The inner of the two layers is the visceral, the outer the somatic 

 layer. In the neighborhood of the ventral mid-line the cells of the 

 mesoderm are rather irregular in form and arranged in but a 

 single layer. At the lateral margins of the mesoderm where the 

 two layers are continuous with one another, they are relatively 

 thick, well defined and separated from one another by a narrow 

 slit-like space, while the component cells of both layers are col- 

 umnar in form. A similar condition obtains in Chalicodoma, and 

 to these lateral regions, composed of columnar cells, Carriere and 

 Burger (1897) applied the name "mesodermal tubes" (Mesoderm- 

 rohre) as descriptive of their form and since these sections of the 

 mesoderm are marked off rather sharply from the remainder both 

 in their form and their behavior, this term will for convenience be 

 adopted in the following description. Between the mesodermal 

 tubes and the single-layered median strip the mesoderm cells are 

 flattened and somewhat irregular in form, particularly those of the 

 visceral layer. In Chalicodoma, Burger (1897) found that the 

 mesodermal tubes were divided intersegmentally by thin parti- 

 tions, moreover, the mesoderm mesiad of the mesodermal tubes 

 is divided into pairs of flat sacs (mesodermal sacs), a pair to 

 each segment, and communicating only intrasegmentally with the 

 mesodermal tubes of the corresponding side. No well marked 

 evidence of segmentation of the mesoderm could be found in the 

 honey bee. The two layers of the mesoderm on each side of the 

 mid-line are virtually continuous from the second maxillary seg- 

 ment to the eleventh trunk segment, although a careful exami- 

 nation of sagittal sections of Stages VIII and IX seems to indicate 

 that the mesoderm mesiad of the mesodermal tubes is slightly 

 constricted intersegmentally. At Stage VIII-IX (Fig. 77, Meso) 

 while the mesoderm has changed but little in its general appear- 

 ance, it may be noted that the cells in the immediate vicinity of 

 the mid-line have increased in size, and assumed a rounded form. 

 These (BIC) are to form the blood corpuscles. 

 At Stage IX (Fig. 78) the mesoderm begins to show important 



