REGENERATION 413 



the new epidermal membranes upon the external surface and lining the 

 canals. The epidermal areas and the more specialized cells of the middle 

 region seem to have no part in the restoration. It is, however, difficult 

 to be sure of what happens if one has recourse only to the common tech- 

 niques of fixation and staining even though checked by study of the living 

 material. According to Wilson (68, page 166) "it has yet to be proven 

 that sponge cells ever dedifferentiate into a regenerative, embryonic 

 condition, although the dissociation phenomena in hydroids lend some 

 support to the view that this is possible"; and again, "the retention by 

 cells of their essential nature, even after complete morphological dediffer- 

 entiation, is illustrated by the dissociated collar cells." 



Conclusive evidence that archaeocytes are essential for reunition in 

 sponges might be obtained by means of the technique of irradiation 

 which appears not to have been employed in the attack upon this prob- 

 lem. From unpublished experiments conducted by the writer upon the 

 reunition and subsequent changes in Microciona prolifera after exposure 

 to X-rays, it appears that such irradiation retards the reunition process 

 and formation of the new sponges ; but the data are inadequate for satis- 

 factory conclusions. The finding by Wilson and Penney (70) of three 

 principal types of cells, which are important in the reunition of sponges, 

 may be significant in relation to the appearance of dermal cells, gastral 

 cells, and archaeocytes, as the primary cell types in the differentiation of 

 certain sponges. A comparative study, by the technique of irradiation, 

 of differentiation in embryonic development, in gemmules, and in the 

 reduction and reunition masses of sponges might yield important data 

 upon the general problem of differentiation as well as data upon this 

 problem in the Porifera. 



HYDROZOA 



In coelenterata the histology of regeneration has proved difficult to 

 follow, and its study has not been undertaken in many species that have 

 been used extensively for investigations upon the general physiological 

 aspects, or what may be called the "organismal" factors in contrast with 

 the histological factors involved. The hydras alone seem to have been 

 extensively studied with regard to these cellular changes. In this family 

 the trend of the recorded observations for many years favored Nuss- 

 baum's account (46) of the interstitial cells as an undifferentiated stock 

 from which all the other cell types arose, both in reproduction and in the 

 normal growth of the animal. The abundance of such totipotent cells 

 explained the great powers of regeneration observed in these animals. 

 On the other hand, Rowley (50), in a brief paper without figures, con- 

 cluded that in Hydra viridis "the new cells which appear during the 

 regeneration of the hydra are formed by division of the old cells through- 

 out the entire piece, as in the normally growing animal." She found 



