184 W. L. TOWER. 



These changes which occur in the cuticula involve only the 

 secondary cuticula and are the same irrespective of what the 

 habitat of the larva may be, as shown in the figures given and 

 result in all in the thinning and weakening of the old integument. 

 All these changes take place in the latter part of the period of 

 preparation and the first third of the following stage. 



After the separation of the cuticula and hypodermis the new 

 primary cuticula begins to form at once. It appears first as a 

 thin delicate lamella spread evenly over the entire outer surface 

 of the hypodermis and grows rapidly in thickness until finally 

 just before ecdysis takes place it reaches its final thickness. It is 

 developed as a delicate structureless membrane secreted by the 

 hypodermal cells and there appears at no time evidence in favor 

 of the oft-repeated statement that the cuticula is the hardened 

 outer ends of the hypodermal cells. After ecdysis this primary 

 cuticula hardens rapidly and develops its coloration through 

 enzyme action precisely as in the adult beetle, a process which I 

 have described elsewhere. 



As soon as ecdysis is over the deposition of the secondary 

 cuticula begins. This layer is, as is well known (Vossler, Tower) 

 a carbohydrate allied to tunicin and is deposited in alternating 

 layers through the periods of reconstruction and growth when it 

 attains its maximum thickness. It is everywhere penetrated by 

 delicate pore canals which are the fine canals occupied by the 

 protoplasmic processes of the hypodermal cells which do not 

 become detached from the primary cuticula until just before 

 ecdysis when they are withdrawn. With poor killing and pres- 

 ervation, however, they are all withdrawn and the canals appear 

 empty, but are not so in life. 



The hypodermis also goes through a regular cycle of changes 

 in the shape of the cells during each of the cycles. These 

 changes are first an increase in the number of cells in the growth 



o *-> 



period, and second changes in shape and arrangement so as to 

 give the body wall the greatest rigidity and strength during the 

 period immediately following ecdysis before the new cuticula 

 hardens. In Fig. 7 I have given a series of stages in semi-dia- 

 grammatic form showing the change in the shape and arrange- 

 ment of the cells. In Fig. 7, A, in the growth and differentia- 



