356 SENESCENCE AND REJUVENESCENCE 



initiated either by the physiological or physical isolation of the egg 

 cell from its source of nutritive supply in the parent body, or often 

 by its extrusion from the body into water, or in many cases only 

 after the spermatozoon has entered the egg. In most cases the egg is 

 incapable of even the maturation divisions, except after some degree 

 of excitation, and in some eggs the isolation from the parent body 

 is sufhcient, while others require the additional stimulation of 

 extrusion into water, and for still others the further change result- 

 ing from entrance of the sperm is necessary. In the formation of 

 the megaspore and microspore in plants and in the spermatogenesis 

 of animals the period of growth between other divisions and matura- 

 tion is slight or practically absent, and with rare exceptions the cells 

 divide equally in the maturation divisions. Whether in these cases 

 also the maturation divisions are initiated by a stimulation of the 

 cells from without is not known, but the probabihty suggests itself 

 that they occur as the result of a physiological or physical isolation 

 of the cells. 



From this point of view the maturation divisions appear to be 

 divisions occurring in relatively old differentiated cells as a reaction 

 to physiological or physical isolation from the parent body, or to 

 this factor in combination with others. Their peculiar features 

 are apparently associated with the low metabohc rate in the cells 

 concerned. In the mosses and ferns the spores resulting from the 

 maturation divisions undergo rejuvenescence and begin a new 

 developmental and vegetative cycle without fertihzation, but in 

 the seed plants the degree of rejuvenescence is apparently slight 

 in most cases and the divisions few in number, and in animals, 

 except in the case of parthenogenic eggs, rejuvenescence occurs 

 only after fertilization. 



CONCLUSION 



In the present chapter the attempt has been made to show that 

 the developmental history of the gametes affords no adequate 

 grounds for the behef that germ plasm is something independent 

 of the rest of the organism. There is no proof of the "segregation 

 of the germ plasm" as an independent entity in embryonic develop- 

 ment, but the germ cells are very evidently determined hke other 



