868 THE PHYSIOLOGY OF REPRODUCTION. 



the two must be taken together to constitute an entirely normal 

 animal. By experiments of this kind it has been shown that cer- 

 tain definite portions of the egg are responsible for the formation 

 of particular organs in the adult. If these portions of the egg are 

 removed the organs in question are not developed. Facts of this 

 kind lead to the evolutionary view that in the fertilized ovum there 

 is a collection of different materials designated as formative stuffs 

 each of which is specific, that is, develops into a special structure. 

 Many facts connected with the regeneration of parts, regeneration 

 of a lost leg in a crab, for example may be used to support a similar 

 view of the existence of specific formative stuffs in the cells of the 

 body.* Wilson has suggested an attractive theory which seems to 

 account for the facts known at present and forms an acceptable com- 

 promise between the extremes of epigenesis and evolution. Accord- 

 ing to him, the germ (fertilized ovum) contains two elements, one of 

 which undergoes a development that is essentially epigenetic, while 

 the other contains a preformed structure which controls and deter- 

 mines the course of development. The first is represented by the 

 cytoplasm of the egg, the second by the chromatin (chromosomes) 

 of the nucleus. The latter have specific structures, and under their 

 influence the nutritive undifferentiated material of the cytoplasm 

 is modified to form specific formative stuffs differing in character 

 in the developing ova of different animals. 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 these theories ; the reader is referred for 

 such information to special treatises on the subject. f 



Determination of Sex. The conditions which lead to the 

 determination of the sex of the developing ovum have attracted 

 much investigation and speculation. In the absence of precise 

 data very numerous and oftentimes very peculiar theories have 

 been advanced. J Such views as the following have been main- 

 tained: that the sex is determined by the ova alone; that it is 

 determined by the spermatozoa alone; that one side (right ovary 

 or testis) contains male elements, the other female; that the sex 

 is a result of the interaction of the ovum and spermatozoon, the 

 most virile element producing its own sex, or according to another 

 possibility "the superior parent produces the opposite sex"; that 

 the sex depends on the time relation of coitus to menstruation, 



* For a discussion of these facts and for various hypotheses see Morgan, 

 " Regeneration." New York, 1901. 



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

 et les grands problemes de la biologie generate, " 1903. 



% For recent accounts of the various theories and discussion see Mor- 

 gan, "Popular Science Monthly," December, 1903; Lenhossek, "Das Problem 

 der geschlechtsbestimmenden Ursachen," 1903. 



