V] THE ALIMENTARY AND EXCRETORY SYSTEMS 111 



can be seen small nests (ns) of developing cells, destined to take 

 the places of those destroyed during the period of activity. These 

 nests were for a long time regarded as being of a glandular nature, 

 and were termed "crypts." 



(ii) The period of preparation (B). After a sufficient period of 

 rest, the larva begins to prepare for another meal. This is marked 

 by the beginnings of a distinct swelling-up of the epithelial cells, 

 owing to the elaboration and storage of digestive ferments within 

 them. The cells become distinctly more elongated, their nuclei 

 sharing in this process. The digestive products collect at the internal 

 or free ends of the cells, in the form of "balls of secretion" (bs). 

 These push the internal border away from the rest of the cell. This 

 border is then seen to consist of a fine continuous membrane, per- 

 forated by numerous very minute pores. As the balls of secretion 

 increase, this membrane is pushed quite clear of the cells. It is 

 then called the peritrophic membrane (pm). Its function is to 

 envelope the food masses received from the gizzard, so as to prevent 

 injury to the delicate epithelium of the mid-gut. If the arrival 

 of the meal is delayed, elaboration of the digestive ferments continues 

 unabated, so that, in the case of a larva that has been fasting for 

 two or three weeks, the mid-gut may be swollen to three or four 

 times its original size, as shewn in c. 



(iii) The period of activity (c). When the food reaches the 

 mid-gut, the epithelial cells are discharged. Some are thrown off 

 whole, with nucleus and contents complete. Others burst and cast 

 out huge balls of secretion, some of which contain only liquid 

 ferments, while others contain the yellow globules originally present 

 in the cells. Thus the yellow colour of the mid-gut is lost after a 

 heavy meal. The cells which are discharged whole mix with the 

 food and are lost altogether. Those which only burst retain their 

 nuclei, and are capable of returning to the quiescent stage, and of 

 reforming their contents again and again, until finally they too 

 become thrown off complete. The digestive ferments may be forced 

 up to meet the food while it is still in the gizzard. 



Regeneration of the Epithelium. The loss of so many cells 

 during the period of activity is quickly repaired on subsidence by 

 the activity of the nests. Their nuclei and cytoplasm divide rapidly, 

 forming numerous cells which move outwards from a common centre, 



