438 BIOLOGICAL EFFECTS OF RADIATION 



the subject of many investigations of the superficial and organismal 

 aspects of the process. The histological aspects, which have received 

 less attention until recent years, have proved extremely difficult to under- 

 stand. According to Hellmich (23, 24) who has made an exhaustive 

 study of these changes, the cells observed in the regenerating limb of an 

 adult salamander, such as Amblystoma mexicanum, can be grouped into 

 two large classes, the hematogenetic and the histogenetic. "The first 

 class includes the erythrocytes, lymphocytes, eosinophile cells, special 

 leucocytes, and plasma cells; the second, the mesenchymal cells, fibro- 

 cytes, wandering cells, mast cells, pigment cells, giant cells, and peri- 

 cytes." In the regeneration of young larvae it is impossible to make 

 any such separation into cell types. It appears, however, that mesen- 

 chymal cells are essential for regeneration in the larval stages, since there 

 is no regeneration following complete extirpation of a limb bud, which 

 presumably results in extirpation of all the cells of the anlage and its 

 immediate environment. Hellmich assumes "that in the normal 

 growth of the limb there remains a residual number of undifferentiated 

 mesenchymal cells," which can then function in adult as well as larval 

 regeneration. While the histological processes of regeneration in adult 

 salamanders are thus very complicated, proceeding from various kinds 

 of differentiated and undifferentiated cells and connected with inflam- 

 matory conditions, the same processes in young larvae are simpler, 

 consisting principally of the multiplication of undifferentiated mesen- 

 chymal cells and differentiation of the cells thus produced without being 

 merely a repetition of embryonic development in a restricted part of the 

 body. 



The studies of Schaper (51) on the effects of radium irradiation upon 

 amphibian development included a brief examination of these effects 

 upon regeneration in Triton larvae. This investigation and the one by 

 Levy (37), who studied Schaper's material after the latter's death, are 

 now important only as confirmed by recent work. Like the work of 

 Bardeen and Baetjer (3) with planarians, these papers constituted impor- 

 tant exploratory studies that might have led to further research as an 

 immediate outcome, since it was shown that such irradiation inhibited 

 regeneration. No one else seems to have undertaken further studies 

 of this sort with vertebrates during many years. Litschko (38, 39, 40) 

 was first in this revival of interest, although closely followed by Butler 

 (7) and by Brunst and Scheremetjewa (6, 52). 



In his latest paper Litschko (40) apparently includes the preliminary 

 accounts given in the two preceding publications and summarizes numer- 

 ous experiments each made with five or six animals and a similar number 

 of controls. He considered the effects of X-rays on regeneration of limbs, 

 tail, and dorsal fin in the axolotl, especially with regard to what caused the 

 inhibition. The exposures, which ranged from 22 to 600 r, were either 



