THE EMBRYOLOGY OF THE HONEY BEE 67 



PMR 



Am 



MP 

 (Meso) 



FIG. 28. Transverse section through the posterior end of an egg of 

 Stage VII, showing the posterior mesenteron rudiment (PMR'), and also 

 the relations of this rudiment and of the lateral plates (LP), 'middle 

 plate (MP}, and amnion (Am}, to each other, x 290. 



ter demands for its production the entire posterior end of the 

 ventral plate, leaving at the sides no undifferentiated blastoderm 

 (ectoderm). In the case of the anterior mesenteron rudiment 

 the ectodermal covering progresses mesiad from the mesial edges 

 of the two halves of the ventral plate bounding the rudiment 

 laterally. In the case of the posterior mesenteron rudiment the 

 ectodermal covering progresses caudad from the posterior limits 

 of the lateral plates, appearing as if they were extended back- 

 ward to cover the posterior mesenteron rudiment. The external 

 surface of the latter is, however, never entirely covered, its 

 posterior end (relative to the embryo) apparently always re- 

 maining uncovered. Moreover the ectodermal covering forms 

 a continuous and unbroken sheet to its caudal limits. There is 

 no break in its continuity corresponding to the future proctodaeal 

 invagination and comparable to the uncovered area of the an- 

 terior rudiment. This difference is of importance, since it fore- 

 shadows the differing relations of the fore- and the hind-intestine 



