The Mechanistic Conception of Life 17 



then it follows that we have two kinds of spermatozoa that differ from 

 each other in a vital matter. We expect, therefore, to find in the off- 

 spring two sorts of individuals in approximately equal numbers, under 

 normal conditions, that exhibit marked differences in structure. A 

 careful consideration will suggest that nothing but sexual characters 

 thus divides the members of a species into two well-defined groups, 

 and we are logically forced to the conclusion that the peculiar chromo- 

 some has some bearing upon the arrangement. 



I must here also point out a fact that does not seem to have the 

 recognition it deserves; viz., that if there is a cross-division of the 

 chromosomes in the maturation mitoses, there must be two kinds of 

 spermatozoa regardless of the presence of the accessory chromosome. 

 It is thus possible that even in the absence of any specialized element 

 a preponderant maleness would attach to one-half of the spermatozoa, 

 due to the ''qualitative division of the tetrads." 



The researches of the following years, especially the 

 brilliant work of E. B. Wilson, Miss Stevens, T. H. Morgan, 

 and others, have amply confirmed the correctness of this 

 ingenious idea and cleared up the problem of sex determina- 

 tion in its main features. 



According to McClung each animal forms two kinds of 

 spermatozoa in equal numbers, which differ by one chromo- 

 some. One kind of spermatozoa produces male animals, 

 the other female animals. The eggs are all equal in these 

 animals. More recent investigations, especially those of 

 E. B. Wilson, have shown that this view is correct for many 

 animals. 



While in many animals there are two kinds of sperma- 

 tozoa and only one kind of eggs, in other animals two kinds 

 of eggs and only one kind of spermatozoa are formed, e.g., 

 sea-urchins and certain species of birds and of butterflies 

 (Abraxas). In these animals the sex is predetermined in the 

 egg and not in the spermatozoon. It is of interest that, 

 according to Guyer, in the human being two kinds of sperma- 

 tozoa exist and only one kind of eggs; in man, therefore, sex 

 is determined by the spermatozoon. 



