272 THE EVOLUTION THEORY 



to tlie comlitions of fertilization. It would be erroneous to suppose 

 that, with the hi',^her ditferentiation of the organism as a whole, the 

 differentiation of the germ-cells became increasingly complex. On 

 the contrary we find even among Algaa, as the case of Fibcus shows, 

 a marked i.lifierence between the sex-cells, which rather decreases than 

 increases among man}^ of the higher plants. It is not on the more or 

 less complex structure of the organism itself that the nature and 

 degree of the dimorphism of the germ-cells depends, but on the 

 special conditions which are involved in each case, both in the union 

 of the two kinds of sex-cells and in the subsec[uent development of 

 the product of this union, the ' fertilized ovum.' 



Thus it comes about that the male or ' sperm-cells ' of the lower 

 plants, of the lower animals, and, again, of the highest animals are 



similar in structure. In all these 

 organisms the male germ-cells 

 exhibit the minuteness, the form, 

 and the activity of the so-called 

 ' zoosperms ' or ' spermatozoa,' 

 that is to say, they are thread- 

 like, very minute corpuscles, 

 which move rapidly forwards in 

 water or other fluid with undu- 

 latory movements, and penetrate 

 Nt^ into the ovum with similar boring- 



movements when they have been 



Fig. 64. Fucus piatycarpus, brown sea- fortunate enough to reach their 

 wrack. £■/, ovum, surrounded by swarming i . i 



sperm-cells {sp). After Schenck. goal. At the anterior end they 



possess a more or less conspicuous 

 thickening, the so-called ' head ' in which the nucleus lies, and this 

 is followed by the ' tail,' a thread-like structure consisting of cytoplasm 

 which effects undulatory movements comparable to those of the 

 flagella of Infusorians and Volvocineai. The whole spermatozoon is 

 thus a specialized ' flagellate cell.' 



When these ' zoosperms ' were recognized as the ' fertilizing- 

 elements ' in higher animals, and when ' sperm-threads ' had been 

 found, not only in all mammals and birds, reptiles, amphibians, and 

 fishes, but even in many ' invertebrates, ' the conclusion was suggested 

 that the function of fertilization might be discharged by this lively 

 motile substance ; for until the eighth decade of the nineteenth century 

 fertilization was still regarded by many as an ' awakening of life ' in 

 the egg. Since life depends on movement, in truth on infinitely fine 

 molecular movements, of which the movement of the whole germ-cell 



