334 SENESCENCE AND REJUVENESCENCE 



factor in senescence, the spermatozoon should be a very young cell, 

 for it is almost without cytoplasm in most cases. I have called 

 attention to various lines of evidence which indicate that both egg 

 and spermatozoon are highly differentiated, old cells (Child, 'n), 

 and Conklin ('12, '13) has expressed himself as in essential agree- 

 ment with this view. 



The process of formation of the gametes in its morphological 

 aspects is very evidently a process of specialization and differentia- 

 tion. The fully developed game tic cells are among the most highly 

 specialized cells, if not the most highly specialized cells of the 

 multicellular organism, but the primitive germ cells from which 

 they arise are minute cells without any morphological structure 

 beyond that common to cells in general, and with a high metabolic 

 rate in short, with all the visible characteristics of embryonic or 

 unspecialized, undifferentiated cells. The process of development 

 of the gametes from such cells is a process of specialization and 

 morphological differentiation of the same sort as that which occurs 

 in other cells of the organism. Morphologically the fully formed 

 gamete certainly bears no resemblance to an embryonic cell. A 

 few figures will serve to emphasize this point. 



In Figs. 123-31 (pp. 317-18) the sex organs and gametes of some 

 of the algae and fungi are shown. The gametes are readily dis- 

 tinguished from the vegetative cells and in most cases appear to be 

 more highly specialized and differentiated than those. Male 

 gametes, the spermatozoids of a few plants from other groups, are 

 shown in Figs. 148-53. Fig. 148 is the spermatozoid of a liverwort; 

 Fig. 149, a horse-tail, Equisetum; Fig. 150, a fern; Fig. 151, a 

 cycad, Zamia; Fig. 152 is the spermatozoid or generative nucleus 

 of the sunflower; Fig. 140 (p. 320) shows the pollen grain of Sil- 

 phium, another composite with the two elongated generative nuclei 

 or spermatozoids, and Fig. 153, a fully developed spermatozoid of 

 the same plant. These male cells are different in various ways, 

 but most of them possess a well-developed motor apparatus of one 

 kind or another. 



The differentiation of the male gamete among the animals is 

 perhaps more uniform than among plants, but there are many 

 animal species with aberrant forms of spermatozoa. Figs. 154-57, 



