CHAPTER VI 



THE RELATION BETWEEN AGAMIC REPRODUCTION AND RE- 

 JUVENESCENCE IN THE LOWER ANIMALS 



THE PROCESS OF AGAMIC REPRODUCTION IN Pldndrid dorotocephdld 



AND RELATED FORMS 



Pldndrid dorotocephdld, like many other species of flatworms, 

 undergoes from time to time a process of agamic or asexual repro- 

 duction, which consists in the separation by fission of the posterior 

 third or fourth of the body from the rest and its development into 

 a new animal. The posterior region which separates is not morpho- 

 logically distinguishable in any way from adjoining regions of the 

 body, yet the separation occurs at a more or less definite level of 

 the body. 



In the course of an extended study of experimental reproduc- 

 tion in Pldndrid I have found that the posterior body region in all 

 except very young animals, while not morphologically distinguish- 

 able as a new individual, is nevertheless clearly marked off physio- 

 logically from the region anterior to it. Along the main axis of 

 the planarian body a gradient in the rate of metabolism exists 

 (Child, '12, '130), the rate being highest in the head-region and 

 decreasing posteriorly to the region where separation occurs in 

 fission: here a sudden rise in rate occurs, and posterior to this 

 point another gradient similar to that in the anterior region. That 

 is, the posterior region of the body, which is separated from the 

 rest by the act of fission, possesses an axial gradient in rate of metab- 

 olism similar to that of the anterior region. In long worms, two, 

 three, or even more of these metabolic gradients may appear, one 

 posterior to the other. These metabolic gradients in the body of 

 Pldndrid appear, not only in the susceptibility of different regions, 

 but also in the differences in the capacity for reconstitution of 

 pieces from different levels (Child, 'nb, J nc). 



The existence of these metabolic gradients in the posterior 

 region of Planaria indicates, as chap, ix will show more clearly, 



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