THE EMBRYOLOGY OF THE HONEY BEE 49 



of the mesoderm. In the rearrangement of the cells which are 

 to form the mesoderm, cell division plays an important part, 

 since cells in division are very abundant in the middle plate dur- 

 ing Stages V and VI. 



In the formation of the mesoderm Apis differs somewhat from 



DS 



Am 



LP 



MP 

 (Meso) 



FIG. 21. Transverse section through egg. Stage V-VI, near anterior 

 end, showing the middle plate (MP} virtually covered by the lateral 

 plates (LP), x 243. 



most of the insects studied. Following Korschelt and Heider's 

 classification (1891-3) the different modes of formation of the 

 mesoderm in pterygote insects may be distinguished as follows : 

 i. The formation of a tube by invagination of a median strip 

 of the ventral blastoderm. This tube, constituting the mesoderm, 

 may be thick-walled and round in section and consequently pos- 

 sessing only a narrow lumen, as in many Coleoptera, or thin- 

 walled and compressed in a dorso-ventral direction, as in Chali- 

 codoma (Carriere 1890). This type is probably the commonest 

 and best known of the three. It is found in representatives of 

 widely separated orders, for example: Hydrophilus (Kowalev- 



