398 THE EVOLUTION THEOKY 



ill fact it l)reiiks up into a loose, Hocculont, deiul, but still coherent 

 mass of tissue. Within tliis thoi'o arises a new intestine, as I have 

 shown in an early work (1M64); ami Kowalewsky and Van Reos 

 have since made us aware ol' the interesting details of this recon- 

 struction, showing that the new intestine arises from definite C(3lls 

 of the old one, which are iiresent in tlii^ larval gut at certain fairly 

 wide distances, and whieli do not share in the general destruc- 

 tion, but remain iilive, grow, and multiply, and I'orni islands of cells 

 in the dead mass. These living islands, continually extending, 

 ultimately come into contact and again form a closed intestinal canal 

 which differs entirely from that of the larva in its form, in its \arious 

 areas, and in its differentiation. In this case thfis(^ formati\e cells 

 of the imago- intestine must have contained the elements wliieh 

 determined their descendants in nundier, power of nuiltiplication, 

 arrangement, and histological differentiation. In other words, each 

 of these cells must contain the dc^terminants of a particular limiteil 

 section of the intestine of the imago. The other cells of the intes- 

 tinal epithelium could not do this, even though they were under 

 I'xactly the ,same conditions, were included in the same intimate 

 cell-aggregate, and had the same nutritional opportiuiities. They 

 lireak up when the formative cells begin to be active, for till tlion 

 the latter had remained inactive, and liad not multiplied, although 

 they lay regularly distributed among the other cells. Whinice, then, 

 could the entire diffen^nce in the behaviour of these two sets of cells 

 arise, if it does not depend on tlie natwe of the f'/ln thcin.sdves, and 

 how could this difference of nature have developed during the racial 

 history of insect-metamorphosis if determinants did not reach the cell 

 from the 'germ-plasm— determinants which conditioned that some 

 cells should be hereditarily modified into the cells of the imago- 

 intestine and others into the larval intestine '( Quite similar pro- 

 cesses have be(^n recently demonstivitcd in regard to the reconstruction 

 oi the larval intestine in other ins(^et-groups. I)t'egen(n' has done 

 this, for instance, lor the water-beetle (Jfijdro/jhiluii /liceus) ; and it is 

 certain that all these ri^eonstructions start from particular cells, which 

 lie indifl'erently between the active cells during the larval . period, 

 and contain the primary constituents for the formation of a section 

 of the intestine, but which only becouK! active when their hitherto 

 living neighbours die and break up. 



The whole of the reconstruction of the external form of the ily 

 takes place in a similar inanner. Not only the limb, the head, the 

 stigmata, but the skin itself is formed anew from imaginal disks. 

 In each of the abdominal segments three pairs of little cell-islands 



