THEORY OF THE GERM-PLASM 431 



with a sperm-nucleus. Everything points to the conclusion that 

 there is a definite hereditary material, and that it has its seat in the 

 chromosomes of the nuclei of the paternal and maternal germ- cells. 



Evidence that the Germ-plasm is 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, cytoplasm, 

 and centrosome, or that the substance of the egg is the actu3.1 

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

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

 essential basis of inheritance is in the chromatin 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 

 outside the ovum, and what enters is the head, which is almost 

 purely chromatin-material, and 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 longitudinaUy split, 

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

 cells 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 Ufe, the chromosomes — 

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

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

 are many differences in detail. 



6. Boveri succeeded in fertilising with a spermatozoon of one 

 species of sea-urchin the cytoplasm of the ovum of another species 

 of sea-urchin bereft of its nucleus, and the larva had only paternal 

 characters. Delage succeeded in rearing normal larvae of sea-urchins 

 from non-nucleated fragments of ova fertilised by normal sperma- 

 tozoa. Loeb succeeded in developing ova of sea-urchin, annelid 

 {Chcefopterus), and mollusc (Lottia), without the introduction of 

 spermatozoa at all. These facts, taken together, increase our con- 



