18 HEREDITY AND SELECTION IN SOCIOLOGY 



of the parent, is the chromatic substance, or, as Weismann has 

 called it, the Keimplasma or germ-plasm. It is always the 

 vehicle of the hereditary qualities, no matter what the precise 

 mode of reproduction may be. 



It may be remarked that the nuclei of the egg-cell and of the 

 sperm-cell have equal biogenetic value. It is a mistake to speak 

 of the egg-cell as female, and of the sperm-cell as male. Each 

 of these germ-cells has a complete equipment of chromosomes, 

 and each may contain the centrosome which plays such an im- 

 portant part in the complicated process of karyoMnesis. As a 

 rule, however, it is the sperm-cell which imports its centrosome 

 into the ovum. There is, however, no opposition or difference 

 between the properties of the two cells, and the nucleus of the 

 sperm-cell can take upon itself the role of the egg nucleus and 

 vice versa. This fact has been demonstrated by Boveri's ex- 

 periment on the egg of the sea-urchin. Boveri succeeded in 

 separating the nucleus of an egg-cell from the surrounding proto- 

 plasm ; he then artificially fecundated the denucleated egg-cell 

 with the sperm of a male sea-urchin. The fertilised ovum — with 

 a sperm nucleus only — proceeded to develop, the embryonic pro- 

 cess pursued a normal course, and a larva was produced, capable 

 of swimming freely. Weismann has concluded from this experi- 

 ment that the nuclei of the two kinds of germ-cells are in every 

 respect equivalent, and that each is complete in itself. 



Before proceeding to consider the germ-plasm in more detail, 

 we must glance at certain other phenomena presented by the 

 process of fertilisation. It has now been ascertained that the 

 phenomenon of fertilisation is essentially bound up with the 

 blending of two nuclei. Fertilisation may, indeed, be described 

 as the mingling of nuclei (amphimixis). A good example of such 

 mixing is seen in the conjugation of unicellular organisms, in 

 which it often happens that two individuals become one, just 

 as the two germinal individualities or gametes — ovum and sper- 

 matozoon — in the Metazoa become very intimately one in ordinary 



