398 THE EVOLUTION THEORY 



in I'aet it lavaks up into a loose, ilocculent, dead, but still coherent 

 mass of tissue. \\'ithiii this there arises a new intestine, as I liave 

 shown in an early Avork (itS64); and Kowalewsky and Van Rees 

 liave since made us aware of the interesting details of this recon- 

 struction, showing that the new intestine arises from definite cells 

 of tlu' old one, which are present in the larval gut at certain fairly 

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

 tion, hut remain alive, grow, and nuiltiply, and form islands of cells 

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

 idtimately come into contact and again form a closed intestinal canal 

 which ditlers entirely from that of the larva in its form, in its various 

 areas, and in its differentiation. In this case those formative cells 

 of the imaofo- intestine must have contained the elements which 

 determined their descendants in number, power of multiplication, 

 arrancrement, and histological differentiation. In other words, each 

 of these cells must contain the determinants of a particular limited 

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

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

 exactly the same conditions, were included in the same intimate 

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

 break up when the formative cells begin to be active, for till then 

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

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

 could the entire difference in the behaviour of these two sets of cells 

 arise, if it does not depend on the nature of the cells themselves, 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 -^vo- 

 cesses have been recently demonstrated in regard to the reconstruction 

 of the larval intestine in other insect-groups. Deegener has done 

 this, for instance, for the water-beetle [Hydrophilus 2yiceus) ; and it is 

 certain that all these reconstructions start from particular cells, which 

 lie indifferently 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 become active when their hitherto 

 living neighbours die and break up. 



The whole of the reconstruction of the external form of the fly 

 takes place in a similar manner. 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 



