936 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



transmission is called germinal when the micro-organism is conveyed in the 

 ovum or the semen, and placental or intra-uterine when the micro-organism 

 reaches the fetus after uterine development has begun, and chiefly through the 

 circulation. Of germinal infections syphilis seems undoubtedly capable of 

 transmission within either the ovum or the semen. The possibility of germinal 

 transmission of tuberculosis has been maintained, but is not fully proven. Of 

 intra-uterine infections there have been observed in human beings apparently 

 undoubted cases of typhoid fever, relapsing fever, scarlatina, small-pox, 

 measles, croupous pneumonia, anthrax, and possibly tuberculosis, syphilis, and 

 Asiatic cholera. It is obvious that neither germinal nor placental inheritance, 

 both taking place through the medium of a specific micro-organism, and not 

 through the modification of germ-plasm, is comparable to inheritance in the 

 customary sense. 



Theories of Inheritance. From early historical times theories of inher- 

 itance have not been wanting. Physical and metaphysical, materialistic and 

 spiritualistic theories have had their day. Previous to the discovery of the 

 spermatozoon (Hamm, Leeuwenhoek, 1677) all theories were necessarily 

 fantastic, and for nearly two hundred years later they were crude. The 

 theories that are now rife may be said to date from 1864, when Herbert 

 Spencer published his Principles of Biology. Since that date they have 

 become numerous. Even the modern theories are highly speculative ; none 

 can be regarded as being accepted to the exclusion of all others by a large 

 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 

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

 theories of heredity. The father and the mother contribute to the child the 

 spermatozoon and the ovum respectively, and within these two bits of proto- 

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

 There is much 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) that come from all parts of the body ; hence each 

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



