336 CAUSES OF VARIATION 



Born succeeded in uniting tadpoles of two different species of 

 frogs, Rana esculenta for the anterior part and R. arvalis for the 

 posterior. This "made-up" animal lived seventeen days. Harrison 

 succeeded in keeping an individual made up of R. virescens and 

 R.palnstris until its transformation from a tadpole into a frog. 1 



In the same way earthworms may be built up of two or more 

 individuals of the same or of different species, and the pieces used 

 in this building up may even be reversed, so that the middle part 

 of the made-up worm may have its posterior part placed anteri- 

 orly, or the reverse. Worms may be made with two heads, with 

 or without a tail, or with two tails, with or without a head, - 

 though the latter, for obvious reasons, can live but a short time. 2 



Ribbert grafted a portion of mammary gland into the ear of 

 the guinea pig, where it grew, and when the pig became pregnant 

 the grafted tissue secreted milk? 



The whole subject of grafting, even more than that of regen- 

 eration, shows the wonderfully persistent nature of differentiated 

 tissue, though it shows also the variety of conditions under which 

 its activities may be discharged. 



SECTION X EVIDENCE FROM THE ORIGIN OF NEW 

 CELLS AND TISSUES 



New tissue may arise in regeneration in three distinct ways : 



1 . It may arise from tissue of its own kind ; that is, the tissue 

 produced in regeneration may arise from the same kind of tissue 

 in the organism. 



2. It may arise, not from like tissue, of which none may be 

 present, but from the same point of origin and in the same 

 manner as during embryonic development. 



3. It may arise from a source entirely different from that of 

 embryonic development, and in this case may be either normal 

 or heterom orphic. 



Examples of the first class are everywhere at hand ; indeed, 

 this is the most ordinary form of regeneration, and many have 

 erroneously supposed it to be the only form. Ordinarily, muscle 



1 Morgan, Regeneration, pp. 184-185. 2 Ibid. pp. 171-173. 



3 Loeb, Physiology of the Brain, p. 206 



