996 THE PHYSIOLOGY OF REPRODUCTION. 



or chromosomes conveying the carriers or determiners of heredi- 

 tary traits belonging to the parental stock of germ plasm. The 

 ovum contains chromosomes that in the same way add hereditary 

 characteristics from the maternal strain of germ plasm. When 

 these two cells unite a new individual is begun, and his hereditary 

 traits are, so to speak, defined, and they constitute a mixture from 

 two strains, but a mixture the composition of which may be in- 

 finitely varied. Modern biologists insist that the cytoplasm of the 

 ovum also conveys certain general hereditary characteristics of 

 a fundmental kind, such as the polarity and symmetry of the future 

 organism and possibly also its pattern or the relative position of 

 future organs. Such a view, it will be noticed, implies at once 

 preformed structures in the gametes and constitutes one form of 

 an evolutionary hypothesis. This view is further supported by 

 the interesting experiments of Wilson.* 



This author has shown that in certain mollusks (Dentalium or 

 Patella), if a portion of the egg is cut off, the remaining portion upon 

 fertilization develops into a defective animal that is not a whole 

 embryo, but rather a piece or fragment of an embryo. Or if the 

 fertilized egg after its first segmentation is separated artificially 

 into two independent cells, each develops an embryo, but neither 

 one is completely formed each is lacking in certain structures and 

 the two must be taken together to constitute an entirely normal ani- 

 mal. By experiments of this kind it has been shown that certain def- 

 inite portions of the egg are responsible for the formation of partic- 

 ular organs in the adult. If these portions of the egg are removed 

 the organs in question are not developed. Many interesting gen- 

 eral theories of heredity have been proposed by Darwin, Nageli, 

 Weissmann, Mendel, Galton, Brooks, and others. It is impossible 

 to give here an outline of all these theories, but a word may be 

 said regarding the work of de Vries and Mendel, which have given 

 rise recently to so much discussion. For fuller information the 

 reader is referred to special treatises on the subject.f According 

 to the well-known views of Darwin in regard to the action of 

 natural selection it was assumed that new varieties and species 

 are formed by the cumulative action of selection upon small 

 fluctuating variations. By this cumulative selection certain 

 variations are preserved and strengthened until they are suffi- 

 ciently marked to constitute a specific difference, the process 

 requiring naturally a long period of time. In contrast with this 

 view de Vries has suggested what is commonly known as the 

 theory of mutations. According to this view the variability in 



* Wilson, "Science," February 24, 1905, for a popular discussion; also 

 "Journal of Experimental Zoology," 1, 1 and 197, 1904, and 2, 371, 1905. 



fHertwig, "The Biological Problems of To-day"; Delage, "L'heredite 

 et les grands problemes de la biologic generale," 1903; Thomson, "Heredity," 

 1908; Conklin, "Heredity and Environment," 1914. 



