622 JOHN BEARD, 



It is not my purpose to give an historical account of all previ- 

 ous work on the origin of the germ-cells. This would take up 

 much space, and, moreover, those who are interested may profitably 

 peruse Born's excellent review, already referred to. Born's only 

 serious omission to my knowledge is absence of reference to Eigen- 

 mann's work. 



Since 1895 the only other researches calling for notice, beyond 

 those of the latter embryologist, are those of Rabl and Wheeler. 

 Their results will be more fully referred to in the sequel. 



Rabl's finds will be of special interest; for, except in two very 

 important points, so far as they go, they are in agreement with 

 my own. 



The present writing therefore, contains no long history of previ- 

 ous researches. The more recent ones have been summarised by the 

 late Prof. Gustav Born, and as to the earlier ones are they not still 

 to be found in the text-books of embryology? 



The doctrines maintained as to the origin of the germ- cells of 

 Vertebrates may be summarised under three headings: 



1) The germ-cells arise from epithelial cells of the mesoderm, and, 

 more particularly, of the peritoneum (Waldeyer, Kölliker, Balfour, 

 Semon, and others). 



2) The germ-cells do not arise from cells of the embryo itself, 

 but they are cleavage products of the ovum (Nussbaum, Eigenmann, 

 and Goette). 



3) The germ-cells are mainly (Rückert), or entirely (van Wijhe 

 and others) segmental in their origin. 



So far as I am aware, no advocate of the third view has distinctly 

 adopted either of Nos. 1 and 2 as a supplement. 



Logically, therefore, No. 3 is not a doctrine of the origin of 

 germ-cells at all, but only of their supposed arrangement at some 

 phase or phases, also not clearly defined, of the development. 



If it were true, and, as the sequel will show, along with Rabl 

 I hold it to be destitute of any real basis of fact, it might be con- 

 joined with either No. 1 or No. 2; and one might say, that there 

 were four possible theories of the origin of the germ-cells of Verte- 

 brates. 



Of course, only theories supposed to be based on observation are 

 meant. Of theories based merely on speculation there might be any 

 number. Just as there are numberless theories of the "origin of sex", 



