REGENERATION 



437 



TrZ 



regeneration only to that extent. The cells that are important in the 

 formation of new parts are certain undifferentiated cells of the meso- 

 dermal region which Brien regards as representing an embryonic reserve 

 to be found in tunicates and in many other animals. 



In a brief study of the effects of radium irradiation upon Clavelina 

 lepadiformis, Weigand (67) has shown that the regeneration can be 

 affected by this means in the same manner as in planarians. A retarda- 

 tion appeared externally in the later appearance of the caps of cells at the 

 injured regions and the slower regenerative growth and differentiation 

 (Fig. 14). The effect increased with longer exposures, but a complete 

 inhibition was not obtained with the maxi- 

 mum ones used in Weigand's experiments. 

 His study of the normal histology of the 

 regeneration was brief and not conclusive, 

 although he observed that regeneration of 

 the branchial sac began without mitoses. 

 The source of the regenerative cells was 

 not clear, but the " Tropf enzellen " of Spek 

 (54) did not participate. Mitoses were 

 observed later in the regenerative cells as 

 differentiation proceeded and mitoses were 

 also observed in "Tropfenzellen." Histo- 

 logical study of the irradiates showed 

 abnormal mitoses which apparently even- 



KIK 



-TrZ 



TrZ 



Fig. 14. — Effect of radium upon 

 regeneration of Clavelina lepadi- 

 formis. K, control. V, irradiate 

 tuated in the degeneration of the injured exposed to radium 7 hr., cut for 



cells. The branchial sac regenerated more ^r'^T mS^TTZs 'laSt 



slowly because of the lesser number of Differentiation has not proceeded 



normal mitoses in the early stages of the -„S^,'» ^^ ':S^:r,Zt. h! 

 regeneration and the appearance of abnor- heart; Kik, brachial basket; RK, 

 mal mitoses after the longer exposures to ru',^^;^^fS:^T?oTf;u?eiler'TFr.!;; 

 the rays, while the number of "Tropfenzel- Weigand, 67.) 

 len " was found reduced after such exposures. 



In view of his limited study of the histology in the normal and the 

 irradiated specimens and the contradiction by Brien of Spek's inter- 

 pretations, which Weigand accepted, it is impossible to draw any con- 

 clusions from these experiments by Weigand, except that the irradiation 

 exercised an inhibiting effect, as in other cases, and that abnormal 

 mitoses were induced. If Brien's conclusions regarding undifferentiated 

 mesenchyme cells are correct, these cells are presumably the ones most 

 affected and the disturbance of their multiplication or their death would 

 explain the retardation of regeneration that Weigand observed. 



AMPHIBIA 



In vertebrates the amphibia have presented the most favorable mate- 

 rial for studies upon regeneration. The urodeles in particular have been 



