REPRODUCTION AND HEREDITY 297 



proportions. The nucleus of the body-cells has therefore essen- 

 tially the same structure as the first ' segmentation-nucleus.' 



Before we pass 011 let us shortly sum up the facts which 

 justify us in regarding the chromosomes as the material carriers 

 of heritable characters. In the first place we notice the extreme 

 care with which the ' nuclear-loops ' are distributed during cell- 

 multiplication among the daughter-cells. There is no other sub- 

 stance either in the protoplasm or in the nucleus the accurate 

 distribution of which is watched over with the same solicitude. 



Again, the 'maturing division' clearly shows that the 

 chromatin possesses a specific vital importance, and the examina- 

 tion of the fertilization-process finally removes all doubt on 

 that point. It is clear that the germ-cells must contain the 

 ' hereditary substance,' for they produce from out of themselves 

 a new organism equal to that by which they were produced. 

 As further the children inherit characteristics from the father 

 and from the mother in equal proportions, the tiny insignificant 

 male germ must of necessity be equivalent to the ovum which 

 far exceeds it in size, and also contain the ' hereditary substance ' 

 of a complete individual. But egg-cell and spermatozoon agree 

 only in the composition of the chromatin elements and in the 

 number of their ' nuclear loops.' If we now add the fact, already 

 known to us, of the strong individual independence of the indi- 

 vidual ' nuclear loops,' we cannot escape the conclusion that the 

 chromosomes are the real carriers of the heritable characters. 



Moreover, it is possible to demonstrate experimentally that 

 the nucleus and not the protoplasm of the germ-cells determines 

 the development of the growing organisms. By shaking we are 

 able to break the mature eggs of a sea-urchin into several parts 

 which are either nucleated or non-nucleated lumps of protoplasm. 

 If we now bring together, in a drop of sea-water, such non- 

 nucleated fragments with male spermatozoa, the latter behave 

 exactly as if they were dealing with the complete ova : they 

 invade the plasm, or, in other words, perform fertilization. 



As the most important part of the egg-cell, its nucleus, is 

 absent we should expect the act of fertilization to remain with- 

 out results ; but, contrary to expectation, this is not the case, for 

 we see that the egg-plasm, which is here only furnished with the 

 sperm-nucleus, proceeds, nevertheless, to division and growth. 



