REJUVENESCENCE IN EMBRYO AND LARVA 425 



and rejuvenescence in the early stages of plant development. The 

 youngest stage physiologically is jirobably earlier in some and later 

 in other plants, as in different animals, but, as j)ointed out in 

 chap. X, certain parts in most plants remain physiologically young 

 for a long time, or indefinitely, and well-marked dilTerentiation 

 and senescence are confined to other parts. 



THE DEGREE OF REJUVENESCENCE IN GAMETIC AND AGAMIC 



REPRODUCTION 



In gametic reproduction the organism begins its life history as 

 a single cell resulting from the union of two highly specialized, 

 old cells, and the earlier part of this history is a period of dediffer- 

 entiation, cell division, and rejuvenescence. In many cases of 

 agamic reproduction also the life history begins with a single cell, 

 but in many others the reproductive body is a cell mass often con- 

 taining various differentiated organs. Evidently in those cases 

 where a single specialized cell is the starting-point, the degree of 

 reconstitutional change involved in the formation of a new indi- 

 vidual is in general greater than where the individual arises from a 

 large mass of cells, for in the latter case some of the cells or organs 

 are incorporated as parts of the new individual with but little 

 change. It has been shown in chap, v, for example, that in Planaria 

 the degree of reconstitution and rejuvenescence varies inversely 

 as the size of the isolated piece: in the large piece, while certain cells 

 may become embr}'onic, these rapidly differentiate and grow old 

 and the total rejuvenescence is slight, while in the smaller piece the 

 cells undergo more change and the total rejuvenescence is greater 

 in amount. In the single cell which gives rise to a new individual 

 the changes are still greater, and the degree of rejuvenescence of 

 the whole must also be greater, because the reconstitutional changes 

 are very extensive and involve the cell as a whole. Moreover, if 

 it is true that the gametes are more highly specialized than single 

 cells which reproduce agamically, we must conclude that the degree 

 of rejuvenescence is in general greater in gametic than in any 

 form of agamic reproduction, that is, in multicellular organisms. 



If, however, the same degree of rejuvenescence occurs in suc- 

 cessive agamic generations, even though it is much less than thai 



