500 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



future adult, another blastomere another portion, and so on. This theory 

 recalls in a refined form the crude theory of Preformation that was advocated 

 during the seventeenth and eighteenth centuries by Haller, Bonnet, and many 

 others, according to which the germ-cell was believed to contain a minute but 

 perfectly formed model of the adult, which needed only to be enlarged and 

 unfolded in growth. The other modern school, in which Oscar Hertwig is 

 prominent, maintains that the fertilized eg<r is isotropous — that is, that one 

 part is essentially like another part — that the architecture of the egg is rela- 

 tively simple, and that growth is largely a reaction of the living substance to 

 external influences. The idea of isotropy is based largely upon the experi- 

 mental results of Pflfiger, Chabry, Driesch, Wilson, Boveri, and the brothers 

 Hertwig, who by various methods and in various animals have found that 

 single blastomeres of a segmenting ovum, when separated from the others, will 

 develop into normal but dwarfed larva? ; that is, a portion of the original germ- 

 plasm is capable of giving rise to all parts of the animal. These results are 

 interpreted to signify that segmentation, instead of being qualitative, is quanti- 

 tative, each blastomere being like all the others. The second theory, like the 

 first, resembles in some degree a theory of the past two centuries, advocated 

 by Wolff and Harvey, and known as the theory of Epir/enesis. According to 

 this there was no preformation in the germ-cells, but rather a lack of organi- 

 zation which during growth, under guidance of a mysterious power supposed 

 to be resident in the living substance, gave place to differentiation and the 

 appearance of definite parts. 



Modern microscopes have revealed no miniature of the adult in the egg, 

 nor has modern physiology found necessary an assumption of extra-physical 

 forces within living matter. With the increase of knowledge the old and 

 crude preformation of Haller and Bonnet and the speculative epigenesis of 

 Wolff and Harvey have given place to the new preformation and epigenesis 

 of the present time, and all modern theories of heredity may be classed in 

 the one or the other category or as intermediate between them. The mod- 

 ern advocates of preformation explain hereditary resemblance by the supposed 

 similarity of all germ-plasm in any one line of descent. The modern advocates 

 of epigenesis, while allowing the necessity of a material basis of germ-plasm, 

 ascribe hereditary resemblance to similarity of environment during develop- 

 ment. 



I r driation. — It is a commonplace in observation that, however close hereditary 

 resemblance may be, it is never absolute; the child is never the exacl image 

 of the parent either physically or mentally. Variations from the parental type 

 may be either <i<-<inirc<l by the offspring subsequent to fertilization or to birth, 

 and hence are to be traced to the action of the environment; or they may be 

 congenital, that is, inherent in the germ-plasm. Although it is not always 

 easy in the case of any one variation to determine to which class it belongs, 

 yet the fact remains that the two classes exist ; and a complete theory of 

 hereditv must recognize and explain congenital variation as fully as congenital 

 resemblance. It is unnecessary to say that the origin of congenital variation 



