REPR OD UCTION. 937 



aifords a ready explanation of numerous facts, but its highly speculative cha- 

 racter, the entire absence of direct observational or experimental proof of its 

 truth, and the demand .that its conception makes upon human credulity, mili- 

 tate against its general acceptance. Weisruann, the promulgator of the second 

 hypothesis, denies altogether the formation of the germ-plasm from the body- 

 tissues of the individual, and maintains its sole origin from the germ-plasm of 

 the parent of the individual. Through the parent it comes from the grand- 

 parent, thence from the great-grandparent, and so may be traced backward 

 through families and tribes and races to its origin in simple unicellular 

 organisms. According to Weismann, therefore, germ-plasm is very ancient 

 and is directly continuous from one individual to another ; the parts of an 

 individual body are derivatives of it, but they do not return to it their repre- 

 sentatives in the form of minute particles. The general truth of Weismann's 

 conception can hardly be denied. 



As to the morphological nature of germ-plasm, two views likewise are held. 

 One school, led by His and Weismann, holds that germ-plasm possesses a 

 complicated architecture ; that the fertilized ovum contains within its structure 

 the rudiments or primary constituents of the various cells, tissues, and organs 

 of which the body is destined to be composed ; and that growth is a develop- 

 ment of these already existing germs and largely independent of surrounding 

 influences. In accordance with this idea, segmentation of the ovum is specifi- 

 cally a qualitative process, one blastomere representing one portion of the 

 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 egg 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 Pfliiger, 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 larvse ; 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 Epigenesis. 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. 



