DEVELOPMENT OF THE ALIMENTARY CANAL. 427 
(pp. 331-2). This is subsequently converted into an epithelial 
sac, the epithelial elements of which are apparently derived 
from three groups of embryonic cells, which can be made out 
in the alimentary canal of the larva; the histoblasts of the 
proventriculus, of the chyle stomach, and the ring at the 
orifices of the Malpighian vessels. 
“It is by no means easy to understand how these groups of 
cells become a portion of the mesenteric sac, but it appears to 
me probable that the proventricular ring originates the anterior 
extremity of the epithelial sac ; the manner in which the cells 
of the chyle stomach form a continuous layer on the inner 
surface of the sac has been already described (p. 332), but the 
greatest difficulty is to understand how the ring at the orifices of 
the Malpighian vessels is related to the mesenteric sac. The 
sac certainly contains both the distal and proximal intestine, as 
well as the remains of the Malpighian tubes of the larva. It 
appears to me probable that the distribution of the tracheal 
vessels is concerned in the process, and that the new para- 
blastic tissue is brought into relation with the ring through the 
medium of the tracheal trunks which supply it. 
It is certain that those branches of the tracheze which are 
connected with the imaginal rudiments preserve an embryonic 
structure, and have an external coat, consisting of very small 
cells, totally unlike the peritoneal coat of the vessels which 
undergo rapid degeneration during the development of the 
pronymph. Hitherto, however, I have entirely failed to sub- 
stantiate by observations the manner in which the Malpighian 
vessels of the larva are related to their successors, the Mal- 
pighian vessels of the imago. 
It may be admitted that the ring of small cells discovered by 
Ganin, and described by Kowalevski, which I will term Ganin’s 
ring, are the rudiments of the Malpighian vessels and meten- 
teron of the imago, as this appears most probable, but I have 
entirely failed to trace the changes which this ring undergoes. 
When the mesenteric sac is fully developed, no traces of 
intestine or Malpighian vessels are seen in sections outside the 
sac, and the whole of the structures concerned are so friable 
