REPR OD UCTION. 499 



majority of scientific workers, and the excuse for introducing them into a 

 text-book of physiology is the hope that a brief discussion of them may 

 prove suggestive, stimulating, and productive of investigation. 



Germ-plasm. — Germinal substance, germ-plasm (Weismann), or, as it is some- 

 times called, idioplasm (Nageli), must lie at the basis of all scientific theories 

 of heredity. The father and the mother contribute to the child the sperma- 

 tozoon and the ovum respectively, and within these two bits of protoplasm 

 there must be contained potentially the qualities of the two parents. There 

 is the strongest evidence in favor of the prevailing view that the nucleus alone 

 of each germ-cell is essentially hereditary, or, more exactly, that the chromatic 

 substance of the nucleus is the sole actual germinal substance. We have seen 

 that the tail of the spermatozoon is a locomotive organ, and that the body of 

 the ovum is nutritive matter. We have seen also that the essence of the 

 whole process of fertilization is a fusion of the male and the female nuclei, or, 

 more exactly, a mingling of male and female chromosomes. Hence most 

 physiologists agree with Strasburger and Hertwig that the chromatic substance 

 of the nuclei of the germ-cells transmits the hereditary qualities. 



As to the origin of the germ-plasm, two hypotheses have been suggested. 

 Spencer, Darwin, Galton, and Brooks have argued in favor of a production 

 of germ-plasm within each individual by a collocation within the reproductive 

 organs of minute elementary vital particles — "physiological units" (Spencer), 

 "gemmules" (Darwin) — which come from all parts of the body ; hence each 

 part of the body has its representative within every germ-cell. This hypothesis 

 affords a ready explanation of numerous facts, but its highly speculative cha- 

 racter, the entire absence of direct observational or experimental proof of its 

 truth, and the demand that its conception makes upon human credulity, mili- 

 tate against its general acceptance. Weismann, the promulgator of the second 

 hypothesis, denies altogether the formation of the germ-plasm from the body- 

 tissues of the individual, and maintains its sole origin from the germ-plasm of 

 the parent of the individual. Through the parent it comes from the grand- 

 parent, thence from the great-grand pa rent, and so may be traced backward 

 through families and tribes and races to its origin in simple unicellular 

 organisms. According to Weismann, therefore, germ-plasm is very ancient 

 and is directly continuous from one individual to another; the parts of an 

 individual body are derivatives of it, but they do not return to it their repre- 

 sentatives in the form of minute particles. The general truth of Weismann's 

 conception can hardly be denied. 



As to the morphological nature of germ-plasm, two views likewise are held. 

 One school, led by His and Weismann, holds that germ-plasm possesses a 

 complicated architecture; that the fertilized ovum contains within its structure 

 the rudiments or primary constituents of the various cells, tissues, and organs 

 of which the body is destined to be composed ; and that growth is a develop- 

 ment of these already existing germs and largely independent of surrounding 

 influences. In accordance with this idea, segmentation of the ovum is specifi- 

 cally a qualitative process, one blastomere representing one portion of the 



