THE BIOLOGICAL BACKGROUND OF GENETICS 199 



Further specialization of sex cells involves, first, the differentiation of 

 gametes into large passive gametes (eggs) and small motile gametes 

 (sperms) and, second, the differentiation of sex individuals, males and 

 females. 



At this point, it is important to emphasize the fact that sex, al- 

 though it appears to be so intimately associated with reproduction, is 

 in no way essential to reproduction. In fact, it is a hindrance rather 

 than a help to the mere multiplication of individuals. Sex is a mechan- 

 ism that has been superimposed upon reproduction and has a function 

 only remotely associated with the latter. In a word, sex is the diversity 

 mechanism, with which we shall deal fully at the proper time. 



The female gamete, or egg, is always relatively large. Even the 

 tiniest of eggs, such as those of placental mammals, are relatively large 

 as compared with tissue cells. The human egg (ovum), for example, 

 has a cubic content of about a thousand times that of the average tissue 

 cell, while the eggs of birds and sharks are thousands of times bulkier 

 than mammalian eggs. This great increase in the size of eggs is dut 

 largely to the accumulation of food material (yolk) that serves to sus- 

 tain the embryo and give it a good start in life. Most eggs are, when 

 full grown, spherical or ovoid in form; and most of them are protected 

 by envelopes of one kind or another. 



Male gametes, or sperms, are highly variable in shape and are rela- 

 tively extremely small and active. Most of the cytoplasm of the sperm 

 is specialized for locomotor purposes. Some sperms have one long 

 tail like a snake (Fig. 47,/), others have two or several tails, and still 

 others have numerous radiating locomotor processes resembling the 

 thorns of a sand burr. In general, a sperm gives the impression of 

 being a highly specialized cell one of whose main functions is that of 

 finding and penetrating the egg. Sperms are thousands, sometimes 

 millions, of times as numerous as eggs, thus making ft more probable 

 that at least one sperm will reach each mature egg. 



When the sperm approaches the egg, it seems to be guided, at least 

 in some cases, by a specific chemical substance given off by the egg. 

 When egg and sperm meet, the latter penetrates the former more or 

 less completely (Fig. 50, a). Sometimes the whole sperm enters the 

 egg, sometimes only the head of the sperm containing the nucleus and 

 a minute amount of cytoplasm. On entering the egg cytoplasm, the 

 sperm nucleus grows at the expense of the latter and becomes nearly or 

 quite as large as the egg nucleus (Fig. 50, c, d, e). The union of egg and 

 sperm is called fertilization, and the product of the union is a zygote. 



