312 THE DEVELOPMENT OE THE NYMPH. 



there is no shortening of the alimentary tube behind the Mal- 

 pighian vessels. 



During these changes the Malpighian tubes become much 

 paler in colour, and lose the nodulated appearance which is so 

 characteristic of them in the larva, whilst their diameter 

 increases considerably. The histolysis and regeneration of 

 the chyle stomach advances rapidly in the pronymph stage, 

 but the histolysis of the remainder of the intestine progresses 

 far more slowly, and there are at this epoch no traces of its 

 regeneration. 



The Histolysis of the Chyle Stomach, according to Kowalevski 

 [146], occurs in the following manner. 'The intestinal canal is 

 entirely emptied in the resting larva, and its epithelium is 

 altered in character ; the protoplasm of the cells becomes homo- 

 geneous, no longer exhibits vacuoles, and the nuclei, instead 

 of lying near their base, approach their free surface. The 

 most important change, however, is the increase in the number 

 of imaginal cells, which now form true imaginal discs. If 

 wc go a step further and examine the transverse section of 

 a newly- formed pupa, the degenerating epithelium has lost its 

 connection with the muscular wall of the tube, but, instead of 

 Ijing free in its cavity, is surrounded by a thick layer of fusi- 

 form cells. The cells of this layer have nothing in common 

 with the epithelial cells of the mid-gut ; they originate from 

 the same cells from which the new wall of the intestine is 

 developed.' 



It is in this view that I differ chielly from Kowalevski. I 

 think it extremely improbable that the fusiform cells originate 

 from the imaginal elements, as they undergo complete histo- 

 lysis in the njmph. Indeed, I am inclined to regard them as 

 the remains of the muscular coat of the proximal intestine 

 (V\. XVIII., Fig. I). 



It will be remembered that the imaginal rudiments are only 

 present in the chyle stomach, and it is difficult to understand 

 the great shortening of the proximal intestine except on the 

 hypothesis that it is invaginated within the chyle stomach. I 

 have not been able to demonstrate this, but it will be seen 



