CONDITIONS OF GAMETE FORMATION 377 



Proceeding now to the last point in our consideration, the devel- 

 opment of the flower is preliminary to the formation of the gamete. 

 The gametophytes develop as parts of the flower (see pp. 320-22), 

 the pollen grains being the male, the embryo sac in the ovule the 

 female gametophyte. Although these gametophytes are much 

 reduced as compared with those of the mosses and ferns, yet their 

 formation in the seed plants, as in the lower forms, is unquestion- 

 ably the result of a specialized agamic reproductive process, i.e., 

 spore formation. The gametophytes arising from the spore are 

 certainly in the seed plants highly specialized organs or organisms. 

 Their development differs widely in the two sexes and in both is 

 very different from anything else in the development of the plant 

 (see Figs. 140, 141, pp. 320, 321). 



It is in these highly specialized organs, or individuals, as we 

 choose to call them, that the gametes are formed. The whole 

 history of the plant leading up to the formation of the gametes 

 is a history of specialization and differentiation of parts, and we 

 have therefore every reason to regard the gametes as among the 

 most highly specialized and differentiated cells produced by the 

 plant. 



CONDITIONS OF CONJUGATION TN THE PROTOZOA 



According to Weismann all protozoa are potentially germ cells. 

 Maupas in his investigations on the ciliates reached the conclusion 

 that conjugation results from internal factors which, during the 

 period of agamic reproduction, bring about a progressive senescence 

 of the stock ending in death unless conjugation occurs. Conju- 

 gation in some way rejuvenates the animals and so makes possible 

 a new series of agamic generations. But the investigations of 

 recent years, as noted in chap, vi, have forced a change in view. 1 



On the one hand, the breeding experiments of Calkins, Enriques, 

 Woodruff , and Jennings have demonstrated that at least some races 

 of Paramecium and other ciliates can be bred agamically for hun- 

 dreds or thousands of generations, and probably indefinitely, 

 without the occurrence of conjugation and without loss of vigor, 

 provided the proper conditions are maintained in the medium. 



1 Among the more important references are those given in the footnotes on p. 136; 

 soe particularly Woodruff, '14, Jennings, '12, '13. 



