272 



THE EVOLUTION THEORY 



to the conditions of fertilization. It would be erroneous to suppose 

 that, with the higher differentiation of the organism as a whole, the 

 differentiation of the germ-cells became increasingly complex. On 

 the contrary we find even among Algse, as the case of Fucus shows, 

 a marked difference between the sex-cells, which rather decreases than 

 increases among many 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 subsequent 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 

 into the ovum with similar boring 

 movements when they have been 

 fortunate enough to reach their 

 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 

 nagella of Infusorians and Volvocineae. 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 



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Fig. 64. Fucus plaiycarpus, brown sea- 

 wrack. Ei, ovum, surrounded by swarming 

 sperni-cells (sp). After Scbenck. 



