CONDITIONS OF GAMETE FORMATION 397 



CONCLUSION 



It is apparently true for both animals and plants that the pro- 

 duction of gametes is associated with certain internal conditions 

 which are characteristic of an advanced physiological age. But 

 since the course of the age cycle may be accelerated, retarded, or 

 reversed by the action of external factors, the formation of gametes 

 in the lower organisms, where the influence of external factors is 

 relatively great, may often appear to be largely dependent upon 

 these external factors. Not only is gamete production a feature 

 of relatively advanced age, but in some cases the physiological 

 age of the egg parthenogenic or zygogenic character apparently 

 depends to some extent on the physiological age of the parent. 



The association of gamete formation with advanced physio- 

 logical age is a fact of great importance, for it indicates that the 

 "germ plasm ' : is an integral physiological part of the organism 

 and that the formation of the gametes is the final stage of a period 

 of progressive development in the reproductive cells. In the earlier 

 stages of the life history of the organism isolated cells or cell masses 

 may react to isolation by dedifferentiation and reconstitution to a 

 new individual, i.e., agamic reproduction occurs. The partheno- 

 genic egg is apparently a cell which has undergone a considerable 

 degree of differentiation as a gamete, but has not lost the capacity 

 to react to isolation by dedifferentiation and reconstitution. And, 

 finally, the zygogenic gamete has attained a stage of differentiation 

 and senescence in which it is no longer capable alone of reacting to 

 isolation, but can undergo dedifferentiation and reconstitution only 

 when fertilization occurs. If there are any cells in the organism 

 which do not contain " undiff erentiated germ plasm," the gametes 

 certainly seem to be among those cells. 



REFERENCES 



ARMBRUSTER, L. 



1913. " Chromosomenverhaltnisse bei der Spermatogenese solitarer 

 Apiden (Osmia cornuta Latr.)," Arch. f. Zellforsch., XI. 



BARY, A. DE. 



1878. " IJber apogame Fame und die Erscheinung der Apogamie im All- 

 gemeinen," Bot. Zeitung, XXXVI. 



