900 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. In the 

 fertilized 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 peculiarities of the father. 

 Such a view, it will be noticed, implies at once preformed structures 

 in the chromosomes and constitutes one form of an evolutionary 

 hypothesis. This view is further supported by the interesting ex- 

 periments of Wilson.* 



This author has shown that in certain molluscs (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 



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

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



