PROCESS OF REPRODUCTION IN ORGANISMS. 21 



The increased metabolic activity of the egg after fertilization 

 has been demonstrated by various investigators and we know 

 that the rate of cleavage increases from the early stages onward, 

 at least up to a certain stage. During this stage of metabolic 

 acceleration the zygote is undergoing rejuvenescence, not simply 

 because the amount of nuclear material in proportion to cyto- 

 plasm is increasing, as Minot maintains, but because the dis- 

 appearance of structural obstacles to metabolism makes more 

 rapid metabolism possible and so permits increase of nuclear 

 material: in other words, the increase of nuclear material is an 

 incident or a result rather than the cause of the rejuvenescence. 

 The increase in metabolic activity following fertilization is 

 probably not continuous but more or less rhythmical in corre- 

 spondence with the periodic changes in nuclear condition. 

 Lyon's observations (Lyon, '04) indicate the existence of such 

 a rhythm. 



The history of the egg is in many respects similar to the history 

 of a gland cell. During the process of loading, the gland cell is 

 growing old and metabolism decreases until finally the loaded 

 cell can do nothing more and is almost entirely inactive. But 

 when the external stimulus, which corresponds to the stimulus of 

 fertilization in the egg, reaches the gland cell rejuvenescence 

 begins. The accumulated inactive substance becomes active, but 

 in the case of the gland cell is simply eliminated from the cell 

 and finally what remains of the cell is young again and capable 

 of a new metabolic cycle. 



It is an interesting speculation as to whether the egg is not 

 after all closely related to the gland cells. If a gland cell should 

 become isolated from the organism under conditions which 

 render continued existence possible and if it should, in conse- 

 quence of the external stimulus, undergo rejuvenescence to such 

 an extent that it lost its special characteristics as gland cell and 

 became more or less embryonic, it would certainly develop into 

 a new individual in much the same manner as the egg. 



In the case of the developing embryo there comes sooner or 

 later a time probably in most cases after the young animal begins 

 to feed, when the inactive structural products of the renewed 

 metabolism begin to make themselves felt as obstacles to metab- 



