THE EMBRYOLOGY OF THE HONEY BEE 209 



In the honey bee, the mesoderm, as described by Kowalevski 

 (1871), Grassi (1884) and the writer, the coelomic cavity is 

 represented by a narrow cleft, near the lateral margins of the 

 mesoderm. Here the splanchnic and visceral layers are well 

 defined and thick, being composed of long epithelial cells, but 

 mesiad of this point these layers become rapidly thinner and are 

 poorly defined. In Chalicodoma (Carriere and Burger 1897) this 

 distinction is even more sharply marked (see p. 197). In the 

 honey bee no coelomic sacs, as such, are distinguishable, the two 

 mesodermal layers on each side being continuous, longitudinally, 

 as well as the cavity bounded by them. In Chalicodoma the thin- 

 walled portion of the mesoderm, lying mesiad of the mesodermal 

 tubes is distinctly divided into segmentally arranged sacs ("meso- 

 dermal sacs"), while Burger (1897) found that the mesodermal 

 tubes were at a certain stage divided by faint partitions into 

 chambers corresponding to the "mesodermal sacs." In the case of 

 this insect therefore well developed coelomic sacs are present, 

 although flattened dorso-ventrally so that the somatic and vis- 

 ceral layers are in contact with one another, except at their lateral 

 margins. Here the cavities of the coelomic sacs are best developed 

 but are on the other hand virtually fused in a longitudinal direc- 

 tion. In the honey bee this fusion has progressed to such an 

 extent that the adjoining walls of the coelomic sacs of each side 

 have been completely lost and their dorsal and ventral walls have 

 become continuous throughout the entire length of the trunk. 



As regards the fate of the different parts of the trunk meso- 

 derm, modern investigators are in fairly substantial agreement. 

 From the outer or somatic layer of the coelomic sacs are pro- 

 duced the trunk muscles and the major portion of the fat body, 

 from the inner or visceral layer are produced the muscles of 

 the mid-intestine and the genital ridges. The heart is formed 

 from cells situated at the external margin of the coelomic sacs, 

 where an angle is formed by the junction of the somatic and 

 visceral layers. The median layer of mesoderm forms the blood 

 cells. 21 



"Nusbaum (1886, 1888), Nusbaum and Fulinski (1906) and Hirschler 

 (1905, 1909, 1909a) claim that a portion of the mid-intestine is formed 

 from this median strip, which is therefore regarded by these investigators 

 as entoderm (see pp. 73). 



V 



