THE GAMETES IN PLANTS AND ANIMALS 357 



parts of the body by correlative factors. Moreover, the course of 

 development of the gametes bears every indication of being a pro- 

 gressive differentiation and senescence, not fundamentally dilTerent 

 from that of other organs of the body, and the fully developed 

 gametes are physiologically old, highly differentiated cells, which 

 are rapidly approaching death and in most cases actually do die 

 soon after maturity unless fertilization occurs. Whatever their 

 significance for inheritance may prove to be, the peculiar features 

 of the maturation divisions are apparently associated with the 

 condition of advanced physiological age and low metabolic rate 

 in the cells where they occur. These cells, whether they are 

 the spore mother cells of plants or the gamete mother cells of 

 animals, are advanced stages of a period of progressive develop- 

 ment and must undergo dedifferentiation and rejuvenescence 

 before they can enter upon a new period of development. In the 

 plants this may occur to a greater or less extent without fertiliza- 

 tion in the development of the gametophyte, but in the gametes of 

 animals, with the exception of parthenogenic eggs, dedifferentia- 

 tion and rejuvenescence occur only after fertilization. 



REFERENCES 



AMilA, K. 



191 1. " tjber die Differenzierung der Keimbahnzellen bei den Kopepoden," 

 Arch.f. Zellforsch., VI. 



Ballowitz, E. 



1886-1908. The following is a partial list of this author's papers on the 

 structure of spermatozoa: "Zur Lehre von der Struktur der 

 Spermatozoen," Anat. Anz., I, 1886; "Untersuchungen iiber die 

 Struktur der Spermatozoen," Arch.f. mikr. Anal., XXXII, 188S; 

 "Fibrillare Struktur und Contractilitat," Arch.f. d. ges. Physiol., 

 XLVI, 1890; "Untersuchungen uber die Struktur der Sperma- 

 tozoen," Arch.f. mikr. Anal., XXXVI, 1S90; "Das Rctzius'sche 

 Endstiick des Siiugetierspermatozoen," Internal. Monalsschr. /, 

 Anal. u. Physiol., VII, 1890; "Untersuchungen iiber die Struktur 

 der Spermatozoen. Die Spermatozoen der Insekten," Zeitschr. 

 f. wiss. ZooL, L, 1890; "Die Bedeutung der \'alentinschen 

 Querbander am Spermatozoenkopf der Saugctiere," Arch. f. .inat. 

 u. Physiol., Anat. Abt., 1S91; "Wcitere Bcobachtungen iiber den 

 feineren Bau der Siiugetierspermatozoen," Zcilschr. f. wiss. ZooL, 

 LII, 1891 ; " Die innere Zusammensetzung des Spcrmatozoenkopfes 



