270 



The Journal of Heredity 



tributed characters have obvious func- 

 tions ; of the real function of sex we know 

 nothing, and in the rare cases where it 

 seems to have disappeared, the organ- 

 ism thrives to all ai:)pearance just as 

 well without it. And in many other 

 cases, especially in plants, where sex 

 is definitely present, it may apparently 

 be almost or quite functionless, as, for 

 example, in the considerable number 

 of plants which are habitually grown 

 from grafts or cuttings, and in which 

 fertile seeds are never set. It is of 

 course impossible to say with confidence 

 that such 'asexual' reproduction can 

 go on quite indefinitely, but the evidence 

 formerly adduced that continued vege- 

 tative reproduction leads to degenera- 

 tion has been shown to be of doubtful 

 validity. Sex, therefore, although it is 

 almost universally found, cannot be 

 said with certainty to be a necessary 

 attribute of living things, and its real 

 nature remains an apparently impen- 

 etrable mystery." 



To understand the efforts made to 

 solve the problem, it is necessary to 

 have a clear appreciation of what sex 

 is, and of the mechanism of sexual 

 rc])roduction. We speak of certain 

 kinds of animals and plants as female, 

 and of other kinds as male: "The 

 fundamental thing about the female sex 

 is that female individuals produce 

 bodies known as egg-cells or ova, which 

 after uniting with a cell of a different 

 character derived from the male, develop 

 into new individuals." This descri])tion 

 fits equally an animal or a plant. 

 " Sujjerficially, egg-cells vary greatly in 

 aijjjcarance ; they may be relatively 

 large, owing to the inclusion of nourish- 

 ing substance or yolk for the developing 

 embryo, or they may be microscopic, 

 as they generally are when no yolk is 

 present. They may have a special 

 protective covering, or may be naked, 

 but apart from these differences, which 

 are so to speak accidental, they are 

 always characterized, in the most vari- 

 ous animals and jjlants, by consisting 

 of a mass of relatively unmodified 

 protoplasm'^ containing a single nucleus. 



"As the distinguishing character of 

 the female is the production of eggs or 

 ova, so that of the male is the production 

 of male germ-cells, which, however, 

 vary greatly in different cases. They 

 are characterized by the fact that their 

 function is to reach an ovum and unite 

 with it in the process of fertilization, 

 as will be described in more detail 

 below. In nearly all animals and in 

 many of the lower plants, the male 

 germ-cells are for this purpose endowed 

 with the power of independent locomo- 

 tion; in animals they are called sperma- 

 tozoa (in the singular spermatozoon, 

 sometimes abbreviated to sperm) and 

 in the lower plants spcrmatozoids. In 

 the flowering plants the male germ-cells 

 are enclosed in the pollen grains which 

 are produced by the stamens of the 

 flower; the pollen-grains have no power 

 of independent movement, but are 

 carried to the neighborhood of the egg- 

 cell in the female flower, or part of the 

 flower, either by wind or by insects, 

 and thence grow out a tube which 

 penetrates to the egg-cell and carries 

 the male germ-cell into contact with it. 

 Although, therefore, there are great 

 differences between the male germ-cells 

 of different organisms, they all agree in 

 one essential feature, they are adapted 

 for reaching in some way the more 

 stationary egg-cell, they unite with it 

 in the process of fertilization, and the 

 zygote so produced proceeds to develop 

 into a new individual. In animals, 

 in which the spermatozoa have the 

 l)ower of independent movement in a 

 fluid, these are commonly more or 

 less tad])ole-.shapcd, consisting chiefly 

 of a 'head,' which contains little else 

 beside the nucleus of the cell from which 

 they have been derived, and a vibratile 

 protoplasmic tail by the motion of 

 which they travel through the fluid 

 in search of the egg-cell. 



"The es.scntial feature of the process 

 of fertilization is the union of the two 

 nuclei contained respectively in the 

 egg-cell and the head of the spermato- 

 zoon. 'I'hc nucleus is a portion of 

 protoplasm enclosed in a thin membrane 



' " Protoplasm is the name given to the substance which is the material basis of all living things. 

 In chemical constitution it resembles white-of-cgg, and cr>nsists of very complex compounds of 

 carl)()n, hydrogen, oxygen and nitrogen, with a smaller amount of sulphur, phosphorus and other 

 mineral elements." 



