1004 THE PHYSIOLOGY OF REPRODUCTION. 



exist as such in the fertilized egg. This view in contradistinction 

 to the evolution theory was designated as epigenesis. Microscopi- 

 cal investigation has demonstrated beyond all doubt that the fer- 

 tilized ovum is a simple cell devoid of any parts or organs resem- 

 bling those of the adult, and the evolution theory in its crude form 

 has been entirely disproved. Nevertheless the controversy be- 

 tween the evolutionists and epigenesists still exists in modified 

 form. For it is evident that in the fertilized ovum there may exist 

 preformed mechanisms or complexes of molecules which, while in no 

 way resembling anatomically the subsequently developed parts of 

 the organism, nevertheless are the foundation stones, to use a figure 

 of speech, upon which the character of the adult structure depends. 

 Such a view in one form or another is probably held by most bi- 

 ologists, since it avoids the well-nigh inconceivable difficulties of- 

 fered by a completely epigenetic theory. If the fertilized ovum 

 of one animal is in the beginning substantially similar to that of 

 any other animal the epigenesist must ascertain what combination 

 of conditions during the process of development causes the egg, 

 in a dog, for instance, to develop always into a dog, and moreover 

 into a certain species of dog resembling more or less exactly the 

 parent organisms. The infinite difficulties encountered by such a 

 point of view are apparent at once. In this, as in other similar prob- 

 lems, experimental work is gradually accumulating facts which 

 throw some light upon the matter and may eventually lead us to the 

 right explanation. It has been made highly probable that the chro- 

 matin material in the nuclei of the germ cells, the chromosomes, 

 constitute the physical basis of hereditary transmission of racial 

 characteristics. In the fertihzed egg, it will be remembered, half 

 of the chromosomes come from the mother and half from the 

 father, and there is good reason for believing that the maternal 

 chromosomes are the bearers of the maternal characteristics, and 

 the chromosomes derived from the spermatozoon convey the 

 hereditary traits of the father. It must not be understood from 

 this statement that the characteristics of the parent are directly 

 transmitted or handed over to the reproductive cells. On the 

 contrary, the ova of the mother are cells lineally descended from 

 the ovum that gave rise to her as an individual, and the struc- 

 tures or determiners in the chromosomes of the ova are not derived 

 from the body of the mother, but are transmitted from the original 

 ovum. By this means there is constituted a physical continuity 

 of the germinal material from generation to generation. The 

 hereditary characteristics contained in the structure of the germ 

 cells are racial — they belong to the stock and are not created anew 

 in each generation. The spermatozoon, so far as it enters into the 

 structure of the fertilized ovum, consists only of chromatin material 

 or chromosomes conveying the carriers or determiners of heredi- 



