MATURATION AND AMPHIMIXIS 435 



Summary. 



The physical basis of inheritance — the germ-plasm — is in 



the chromatin of the nucleus of the germ-cell. 

 The chromatin takes the form of a definite number of 



chromosomes (or idants). 

 The chromosomes consist of ids, each of which contains a 



complete inheritance. 

 Each id consists of numerous primary constituents or 



determinants. 

 A determinant is usually a group of biophors, the minutest 



vital units. 

 The biophor is an integrate of numerous chemical molecules. 



Maturation and Amphimixis. — It is necessary here to inter- 

 polate a reference (a) to the facts of maturation — the processes 

 that occur in the immature egg-cells (oocytes) and in the im- 

 mature sperm-cells (spermatocytes) ; and (b) to the facts of 

 amphimixis or fertilisation — the intimate and orderly union of 

 the (reduced) nuclei of the two kinds of sex-cells. 



Since the spermatozoon is known to bring into the mature 

 ovum the same number of chromosomes as the mature ovum 

 contains in its nucleus, each act of fertilisation would double the 

 normal number of chromosomes if there were not some process 

 obviating this. The doubling of the normal number does not 

 occur, because the mature spermatozoon and the mature ovum 

 have already undergone a reduction of the number of their 

 chromosomes to half the normal number. 



In various ways, during the divisions of the sperm-cells ante- 

 cedent to their complete differentiation, and during the process 

 which is called the maturation of the ovum — the two divisions 

 which result in the liberation of two polar bodies —the normal 

 number of chromosomes is reduced by a half. Thus, when 

 fertilisation occurs, the number of chromosomes is restored to the 

 normal. This fact has been securely established by the researches 

 of Van Beneden, Oscar Hertwig, Boveri, Henking, and others. 



Reducing Divisions. — Since Van Beneden discovered that 



