68 THE EMBRYOLOGY OF THE HONEY BEE 



to the mid-intestine in the larva, when communication between 

 the mesenteron (mid-intestine) and hind-intestine is completely 

 cut off. 



The source of the posterior mesenteron rudiment is obviously 

 the posterior end of the ventral plate ; its mode of origin is 

 much more clear than in the case of its counterpart at the an- 

 terior end. In the earlier stages of the formation of the pos- 

 terior mesenteron rudiment, as in the anterior rudiment, mitotic 

 figures are rare, so that cell division cannot be regarded as an 

 important factor in the earlier stages of its formation. The 

 posterior end of the ventral plate shares in the general de- 

 crease in breadth associated with the formation of the germ 

 layers, moreover, it also contracts in a longitudinal direction. 

 Decrease in the length and breadth of the ventral plate at its 

 posterior end together with a corresponding elongation of its 

 component cells may then be safely set down as the factors first 

 concerned in the formation of the posterior mesenteron rudi- 

 ment. During the later stages (VI-VII) cell division becomes an 

 increasingly important factor in its growth, as in the anterior 

 rudiment. At Stage VII mitotic figures are exceedingly abund- 

 ant, as figures 27C and 28 show. 



In conclusion then it may be said that both the anterior and 

 the posterior mesenteron rudiments are formed from the blasto- 

 derm of the ventral plate by a movement inward of its cells. In 

 the case of the anterior rudiment the migration takes place by 

 the detachment of cells from a limited area in the mid-line. The 

 rudiment is afterward covered over either by a movement of 

 the lateral halves of the remaining blastoderm toward the mid- 

 line, or by additions to their mesial borders from the superficial 

 cells of the rudiment itself. In the case of the posterior rudi- 

 ment the entire posterior end of the ventral plate is involved, 

 being moulded by changes, first in the form and next in the 

 arrangement of its cells, to form the posterior mesenteron rudi- 

 ment. The ectoderm, which later covers it, is continuous with 

 the posterior ends of the lateral plates, and appears as a caudad 

 extension of them, although it cannot be stated with certainty 

 whether they are formed by an actual caudad extension or by 

 the addition of new material from the external surface of the 

 rudiment. 



