REPRODUCTION 73 



given species. Hence in man we would expect 

 the number of chromosomes in the mature egg 

 cell to be twenty-four. 



In the operation of reproduction the father 

 contributes a spermatozoon which, when it 

 enters the egg, carries with it a negligible 

 amount of protoplasm, a minute body called 

 a centrosome, and a group of chromosomes 

 essentially like those from the mother, and 

 about equal to them in number. Thus the fer- 

 tilized egg consists of protoplasm almost en- 

 tirely from the mother, of a centrosome from 

 the father, and of chromosomes in about equal 

 numbers from each parent. If children always 

 took after their mother, we might suspect that 

 part of the fertilized egg which came exclusively 

 from the maternal side, namely, the protoplasm, 

 to be the bearer of hereditary traits. If they 

 regularly took after the father, we might, with 

 equal right, suspect the centrosome as the 

 organ of transmission. But as children take 

 more or less after both parents and as the only 

 parts of the fertilized egg which are derived 

 from both sources are the chromosomes, 

 these bodies are believed to be the means by 

 which inheritance is accomplished. If this 

 opinion is correct, one must conclude that all 



