THEORY OF THE GERM-PLASM 427 



with a sperm-nucleus. Everything points to the conclusion 

 that there is a definite hereditary material, and that it is in part 

 at least bound up with the chromosomes of the nuclei of the 

 paternal and maternal germ-cells. 



The Germ-plasm mainly Nuclear. — No one can doubt 

 that a germ-cell is a unity, that it represents a " cell-firm," that 

 its virtue is dependent on the interaction of nucleoplasm, cyto- 

 plasm, and centrosome, or that the substance of the egg is the 

 actual building-material out of which the embryo is constructed. 

 And yet, there are many facts which compel us to conclude that 

 the basis of inheritance is essentially bound up with the chromo- 

 somes of the nucleus. Repeating, in part, what we have said 

 in Chapter II., we may note the following facts : 



1. In some cases almost the whole cytoplasmic differentiation 

 of the spermatozoon — namely, the locomotor apparatus — is left 

 ou'.side the ovum, and what enters is the head, which is almost 

 purely chromatin-material, plus the minute mid-body or centrosome, 

 which functions as a dynamic centre in division. 



2. The chromatin-bodies or chromosomes have a constant number 

 for each species, except that in the mature sex-cells the number is 

 half the normal, i.e. half the number found in the body-cells. 



3. In nuclear division the chromosomes are longitudinally split, 

 and are in various ways so distributed that each of the daughter- 

 ceils into which a mother-cell divides receives a precisely equivalent 

 quota of chromosomes. 



4. In many cases it is certain that the chromosomes of the sperma- 

 tozoon entering the ovum are precisely equivalent in number to 

 those which the mature ovum contains. 



5. Throughout the whole world of life, the chromosomes — 

 whether during the growth, or the maturation, or the amphimixis 

 of germ-cells — behave in a generally similar manner, though there 

 are many differences in detail. 



Ancestral Plasms.— Assuming that the chromatin substance 

 cf the nucleus of the germ-cell is the vehicle of the inheritance, 

 Weismann argued that it " contains not only the primary con- 

 stituents of a single individual of the species, but also those 



