HTPODERMIS. 379 



during the course of these processes. Such transformation affects 

 especially the hypodermis, the intestinal canal, the muscles, the fat- 

 body, and the salivary glands. The modification of the tracheal 

 system can only to some extent be classed in this category ; other- 

 wise it appears to be due' to simple regeneration through division 

 of the cells. The heart, the central nervous system, and the genital 

 rudiments undergo slighter alteration. The changes that take place 

 in the different organs must now be discussed separately. 



Hypodermis. 



The hypodermis of the imago arises through an extension of the 

 ectodermal portion of the imaginal discs. This has already been 

 stated in connection with the thorax (p. 375). While the limbs 

 of the thorax gradually attain development in the pupa, a hypo- 

 dermal layer, consisting of numerous small cells, extends from their 

 points of insertion ; this layer, which apparently arises from the 

 peripodal membrane, spreads more and more over the surface of the 

 pupal thorax, while at the same time the area of the large-celled 

 larval hypodermis is correspondingly more and more circumscribed. 

 The flat edges of the newly-formed hypodermis (Figs. 187, hi and 

 188, i) grow into the slit between the superficial cuticle and the 

 larval hypodermis (Fig. 188, h), so that, at these points, the old 

 hypodermis which is undergoing disintegration comes to lie on the 

 inner side of the newly-formed epithelial layer (Fig. 188 B). It is 

 thus evident that, during the substitution of the new for the old 

 hypodermis, the continuity of the superficial epithelium does not 

 anywhere appear interrupted. Since the edges of these two hypo- 

 dermal envelopes overlap, there is not anywhere a point of the 

 body-surface devoid of epithelium. The disintegration of the larval 

 hypodermis is accomplished by the action of the leucocytes (Fig. 

 188, A - ), which become massed in the neighbourhood of the dis- 

 integrating hypodermal cells and ingest the latter. Since the 

 assimilated fragments assume the shape of rounded granules, the 

 leucocytes may now be distinguished by the name of the granular 

 spheres (Weismaxn). The granular spheres, which abound in the 

 future body-cavity of the pupal stage, are therefore nothing more 

 than leucocytes (blood-corpuscles) which have assimilated the dis- 

 organised tissue of the disintegrating larval body. It should here 

 be noted that the breaking-up of the larval tissues is not preceded by 

 the death of the cells, but is the result of the action of leucocytes on 

 the still living tissues which have lost their active functions. While 



