REJUVENESCENCE IN EMBRYO AND LARVA 407 



evident increase in activity in parthenogenic eggs during and after 

 maturation leave no room for doubt that the physiological changes 

 which occur in zygogenic eggs after the entrance of the sperm occur 

 in parthenogenic eggs independently of the sperm. Moreover, 

 among animals most parthenogenic eggs undergo only one matura- 

 tion division before beginning development. It was also pointed 

 out in chap, xiii that in many cases parthenogenic eggs are appar- 

 ently less highly differentiated morphologically, and younger phys- 

 iologically, than zygogenic eggs of the same species. 



The obvious conclusion in the light of the various facts is that 

 eggs which are capable of parthenogenic development in nature are 

 less highly specialized as game tic cells than those which require 

 fertilization. They react to isolation by undergoing dedifferentia- 

 tion and reconstitution into new individuals, and in this respect 

 they resemble the pieces from the bodies of many lower animals, 

 such as Planaria, which undergo reconstitution when isolated. 

 The capacity of parts of the body for reacting to physiological 

 or physical isolation by dedifferentiation varies inversely as the 

 degree of physiological stability of the structural substratum (see 

 pp. 39-42). But physiological stability of the substratum appar- 

 ently increases during individual development and also during the 

 course of evolution, and often varies to a considerable extent in 

 related species. Since the development of the primitive egg cell 

 into an egg is apparently subject to the same laws as the develop- 

 ment of other parts of the body, the parthenogenic egg must repre- 

 sent an earlier stage of development than the zygogenic egg of the 

 same species. But it does not by any means follow that the eggs 

 of all species would develop parthenogenically if they were iso- 

 lated at a sufficiently early stage. Since the bodies of different 

 species and the different tissues of the same individual possess very 

 different degrees of reconstitutional capacity, we must expect to 

 find differences of the same sort in eggs. Moreover, since the 

 formation of gametes is characteristic of relatively late stages in 

 the individual life history, we should expect a rather high degree 

 of physiological stability in the eggs of most species and partheno- 

 genesis in comparatively few. As a matter of fact, parthenogenesis 

 occurs only here and there among organisms, but it is of interest to 



