PROCESS OF REPRODUCTION IN ORGANISMS. 9 



segmentation and reduplication of parts for these are manifestly 

 merely cases of partial reproduction. 



In all of these cases physiological isolation is followed by a 

 more or less complete reconstitution, according to the degree and 

 character of the isolation. The result of the physiological isola- 

 tion is then essentially the same as the result of physical isolation: 

 the isolated part undergoes first a dedifferentiation and loses more 

 or less completely its original characteristics as a part and then 

 redifferentiates into a whole, or in cases of segmentation and 

 reduplication of parts, into a new part. In the cases which are 

 commonly regarded as reproductions the physiological isolation 

 is usually followed sooner or later by complete physical isolation. 



We have now to ask the question whether rejuvenescence 

 occurs in asexual reproduction in nature. For certain forms, 

 e. g., Planaria (Child, 'n&) and some ccelen cerates, I have deter- 

 mined experimentally that rejuvenescence does occur in asexual 

 reproduction. As regards the plants there can be no doubt that 

 the same is true : the bud and the new organism arising from the 

 spore both represent conditions nearer the starting point of the 

 developmental cycle than the individuals of which they originally 

 formed a part. If my conclusions stated above concerning the 

 nature of senescence and rejuvenescence are correct there can 

 be no doubt that in every case where a dedifferentiation and 

 redifferentiation follow the physiological isolation of a part a 

 greater or less degree of rejuvenescence occurs. 



And finally, there is no more reason to assume the continuous 

 existence of a hypothetical germ plasm in asexual reproduction 

 in nature than in the case where reconstitution follows the 

 experimental physical isolation of a part. After isolation the 

 part may return to or approach the condition of undifferentiated 

 totipotent protoplasm, but the assumption that undifferentiated 

 protoplasm exists in it continuously is wholly gratuitous and 

 superfluous. 



IV. SEXUAL REPRODUCTION. 



We turn now to the problem of sexual reproduction. Here a 

 number of questions require consideration. First, do the gametes 

 arise from an undifferentiated germ plasm which is independent 



