196 



REGENERATION 



anterior end. It meets the anterior end of the digestive tract ; the 

 two fuse, and the communication of the digestive tract with the out- 

 side is established. The pharynx develops from the anterior part 



of the digestive tract, which 

 after Hescheler's operation 

 may contain some of the origi- 

 nal ectodermal stomodaeum, 

 since only five of the anterior 

 segments were cut off, and 

 the embryonic stomodaeum 

 extends somewhat behind this 

 region. In another experi- 

 ment, carried out by Kroeber, 

 somewhat more of the anterior 

 end was removed, but the re- 

 sult was the same (Fig. 59), so 

 that it is clear that the new 

 pharynx may be formed from 

 the old endoderm. 



Hescheler leaves several 

 points still unsettled, more 

 especially the origin of the 



FIG. 59. After Kroeber. Regeneration of anterior cells that give rise to the new 

 end of Allolobophora fastlda. after removal of six > , !_*.*. i 



segments. The first stomodoeal invagination mUSCUiature, but it IS almost 



had been destroyed. The new pharynx is devel- impossible tO make OUt their 

 oping from the endoderm. ... 



origin in this animal, owing to 



the presence of the lymph cells. Hescheler's discovery that the cells 

 of the lymph plug do not themselves, in all probability, contribute to 

 the new part, is an important result, and shows that these seemingly 

 undifferentiated cells do not possess the power of giving rise to the 

 different kinds of new tissues. The in-wandering of cells into this 

 solid plug from the ectoderm, and perhaps also from other sources, 

 and their subsequent union to produce the definitive organs, is also a 

 point of capital importance, especially as it puts us on our guard 

 against a too ready acceptation of the view that all cells in a mass 

 that have the same general and undifferentiated appearance have had 

 a similar origin, and in showing that apparently indifferent cells may 

 really carry with them into the new part those characters that deter- 

 mine their fate. Other cells, apparently equally undifferentiated, 

 and lying in the same position, may have quite different possibilities. 

 In the vertebrates, the regeneration of the tail and limbs of am- 

 phibia and of the tail of lizards has been studied by a number of 

 investigators. The regeneration of the tail of several urodeles 

 and of the larva of the frog was investigated more fully by Fraisse 



