PROCESSES RELATED TO SEXUAL REPRODUCTION 231 



animal. When ovum and spermatozoon unite in fertilization, the 

 full or diploid number is restored by union of these two haploid 

 groups. It will be recalled that the chromosomes of many cells 

 are present in pairs (c/. p. 234). The members of such a pair in 

 the cells of an adult animal are presumed to have descended, one 

 from the ovum and the other from the spermatozoon, by this union 

 of the two haploid groups in fertilization. IMaturation might be 

 characterized as a device whereby the number of the chromosomes 

 is prevented from being doubled in each generation at the time of 

 fertilization. It is believed to occur with minor variations in the 

 germ cells of all metazoa, and comparable phenomena preceding 

 conjugation can be recognized in many of the protozoa (cf. Fig. 110). 

 As similar reductions in the number of chromosomes in the gametes 

 and restoration of the diploid number by fertilization occur in 

 plants, the phenomenon is well-nigh universal. 



The Gametes. — The foregoing account of the history of the 

 female germ cells explains the origin of the ovum. In contrast 

 to the spermatozoa the distinctive characteristics of ova are the 

 additions of nutrient material in the cytoplasm and of cell mem- 

 branes that may be variously developed. Some ova are amoeboid 

 and can migrate for short distances by means of pseudopodia-hke 

 processes; but in most instances the egg of a metazoan is a non- 

 motile, food-laden cell, comparable with the macrogamete of a 

 protozoan and hence markedly different from the spermatozoon. 

 The ovum is, indeed, a rather typical cell and easily recognizable 

 as such (cf. Fig. 210, p. 401). 



The history of the male germ cells likewise explains their cellular 

 nature, but the mature spermatozoon is a much more speciahzed 

 cell than the ovum (Fig. 115 A). In a schematic representation of 

 a type that occurs in many vertebrate animals (Fig. 115 D),the fol- 

 lowing parts can be recognized. The " head," as it was called by 

 the early microscopists in contrast to the " tail," consists of 

 a nucleus composed principally of very dense chromatin and 

 surrounded by a thin layer of cytoplasm. This cytoplasm is 

 continuous with the " middle-piece," which contains a centriole. 

 The " tail," or flngellum, contains an axial filament which extends 

 into the middle-piece to a point near the centriole and is sur- 

 rounded, except at its posterior end, by a sheath of c\i:oplasm. 

 These peculiarities of structure do not appear in the male cells until 

 the last stages of their development, following maturation (Figs. 



