412 BIOLOGICAL EFFECTS OF RADIATION 



it may be supposed that the inhibition of regeneration by radium and 

 X-rays results from injury or destruction of these "embryonic" cells 

 without which the regeneration cannot occur. To test such an hypoth- 

 esis, one should inquire regarding the existence and the roles of cells 

 with embryonic potencies in each particular case where the regeneration 

 can be thus inhibited, and whether these cells are affected by the rays in 

 question. 



There are in theory four possible sources for new tissues in a regen- 

 erate: like cells may produce like; specialized cells may dedifferentiate 

 and then redifferentiate into other cell types; unspecialized, "embryonic," 

 or "formative" cells that have persisted from the early stages of the 

 individual's development may be the source of new tissues; or the changes 

 may include more than one of these processes. In the papers of Kor- 

 schelt (32), Goetsch (19), and Hellmich (23) will be found accounts of 

 this histological aspect of regeneration and many references. To review 

 these histological relationships and the effects of irradiation, as described 

 in particular cases, reference will be made to some of the recent papers 

 which deal with the histology of regeneration in the various phyla and to 

 studies of regeneration that have been undertaken with the technique 

 of irradiation. 



PORIFERA 



The conditions in Porifera may be cited, although it can be said that 

 true regenerative processes do not occur in sponges because the members 

 of this phylum have no specialized organs that can be removed and then 

 restored. Some genera, such as Spongilla, will produce new functional 

 sponges from cuttings, but many others do not have this power, although 

 healing and formation of some new tissue undoubtedly occur. On the 

 other hand, the reduction bodies formed by certain species (68, 69, and 

 47) have long been known to "regenerate" new sponges, and sponge 

 gemmules develop in a manner that resembles regeneration. The forma- 

 tion of new sponges by the reunition masses produced from dissociated 

 sponge cells should be mentioned, although such a process may be 

 regarded as too extreme a reorganization to be called regeneration. In 

 the case of reunition, recent histological studies (68, 70) indicate that 

 dedifferentiation and redifferentiation into cells that may be of another 

 type do not occur in the reassociation and subsequent changes by which 

 the new sponge is formed, and this seems equally true in reduction and 

 the formation of a new sponge that follows (47). The only cells that 

 survive in numbers are the choanocytes, which lose their collars and 

 flagella temporarily but become normal choanocytes again in the newly 

 forming mass; and the archaeocytes, one type of which, the nucleolate 

 cells, can form all the other cell types including choanocytes; while 

 another type, which includes smaller and more active cells, forms only 



