6o4 a. S. Jennings 



pieces of substance are not necessarily permanentiy differentiated 

 to play certain parts. The organism is plastic, and is made over 

 at fission. The normal reproduction involves the same v^ork- 

 ing over and re-differentiation — "morphallaxis" — that occurs in 

 regeneration. 



3 Yet this making over is not complete. Oral and aboral 

 surfaces retained their relative position throughout these tv^entv- 

 two generations, the spine remaining always on the aboral sur- 

 face. Furthermore, the entire history shows that a given struc- 

 ture maybe bodily transmitted for many generations without becom- 

 ing greatly changed. It may even, finally, acquire a more or less 

 permanent position, remaining for at least several generations. 



In the normal reproduction we find structures which behave in 

 both of these ways — some being directly transmitted, others 

 re-made. The two contractile vacuoles of Paramecium pass 

 bodily, one to each of the progeny — though each individual forms 

 likew^ise one new one. The mouth and pharynx are said to pass 

 to the anterior product of fission, the posterior product forming 

 new ones. The oral groove, the blunt anterior and the pointed 

 posterior end, these are examples of structures that disappear in 

 reproduction and are made anew. The cilia and setae of the Hypo- 

 tricha are not transmitted, but produced anew in the new individ- 

 uals. Fission is on the whole mainly a process of reorganization 

 and new production, rather than of transmission. 



J Fate of Other New Structures in Reproduction 



The fate of many other new structural peculiarities was fol- 

 lowed in various individual lines; after the detailed account we 

 have given above, these can be set forth briefly. 



a Spines, Points or Appendages 



In many cases studied the history of points or appendages on 

 the body differed from what we have described above for the line a. 



I This is the case with the small point on a^S already mentioned. 

 (Fig. 4, ^). As will be recalled, there resulted from the division 

 of a^ two individuals bearing spines or points; w^e have followed 



