40 



Now, as to their origin, whence are they ? Referring to 

 any recent text book on the subject, we shall learn that eggs 

 and sperms are derived from certain cells known as primordial 

 germ-cells. That these primordial germ-cells have their origin 

 in a germinal epithelium, which in the higher animals is a 

 specialized region of the peritoneal epithelium, and consists of 

 two more or less pronounced ridges. These ridges ultimately 

 form part of the genitalia. 



We shall learn further that the primordial germ-cells are 

 sexually indifferent, Le., neither male nor female, and that the 

 chief factor in the determination of sex is the external stimulus 

 provided by nutrition, high feeding producing females for the 

 most part, while spare diet tends to the production of males. 

 There are also many other causes regarded as determining sex. 



We shall also learn that from the zygote — fertilized 

 egg — is developed an embryo. With regard to the method of 

 this development, two rival theories at the moment hold the 

 field — Epigenesis and Evolution. 



Epigenesis regards the development as " a new formation 

 of complexity," receiving its control from without, i.e., the 

 development of its various organs, tissues, etc,, is due to 

 environment. This view is championed by Oscar Hertwig and 

 other eminent biologists. 



The other, Evolution, an unfolding, or as its opponents are 

 pleased to miscall it, preformation, contemplates the develop- 

 ment as being controlled from within, i.e., it regards the zygote 

 as containing the potencies of the adult, and is in a sense " the 

 becoming visible of complexity previously invisible to us." 

 This invisible complexity must not, however, be regarded as a 

 miniature organism as did the preformationists of old. but 

 rather as of organized material bearing the latent potencies of 

 the adult, which is a very diflferent thing. 



This view Weismann has elaborated into a very complete 

 and fascinating theory ; he recognizes in his Germ-plasm theory 

 all the forces necessary for the explanation of " Heredity," and 

 " Evolution " in its wider sense. 



