222 THE EMBRYOLOGY OF THE HONEY BEE 



the bowl, extends over the dorsal surface of the yolk toward the 

 caudal pole of the egg. Meanwhile the posterior mesenteron 

 rudiment has also become altered in form. At Stage VII (Fig. 

 27C, PMR) it is disk-shaped, but between Stages VII and VIII 

 it rapidly elongates, sending out a thin tongue-like process over 

 the dorsal region of the egg toward the cephalic pole (Fig. 87B). 

 At the same time the ectoderm grows caudad (with respect to the 

 embryo) around the caudad pole, covering over that part of the 

 posterior mesenteron rudiment which lies closest to the caudal 

 pole of the egg (compare Figs. 27C and 87B, PAIR). With the 

 development of the tongue-like extension of the posterior mesen- 

 teron rudiment the rudiment as a whole becomes thinner, espe- 

 cially in the mid-line. The relatively rapid changes in form under- 

 gone by both mesenteron rudiments during Stages VII and VIII, 

 appear to be due principally to changes in the form and arrange- 

 ment of their component cells, and not to an increase in their 

 number or volume. 



In the example of Stage VIII represented by figures 87A and B, 

 the anterior mesenteron rudiment extends over the dorsal surface 

 of the yolk about one-third of the length of the egg. The thin 

 tongue-like process of the posterior rudiment on the other hand 

 extends towards the cephalic pole nearly one half of the length 

 of the egg. The opposite edges of the two rudiments are therefore 

 not far apart. During the interval between Stages VIII and IX 

 both mesenteron rudiments continue to spread over the dorsal 

 surface of the yolk until their opposite margins meet and fuse 

 in the dorsal mid-line. The point of meeting is usually nearer 

 to the cephalic than to the caudal end of the egg, on account of 

 the more rapid growth of the posterior rudiment along the median 

 dorsal surface of the yolk. By Stage IX the cells of the united 

 mesenteron rudiments have become distributed in a single layer 

 over the dorsal surface of the yolk (Fig. 78, Mint. 88A, AMR), 

 The cephalic end of the yolk is also covered by the epithelium 

 of the anterior mesenteron rudiment, part of which forms the 

 floor of the stomodaeal invagination, as shown in figure 88A, 

 AMR. The caudal end of the yolk on the other hand is still 

 uncovered (Fig. 88B, PMR). The thickness of the newly formed 

 epithelium of the mesenteron is at first not uniform, being greatest 



