The germ-cells. 621 



Certainly, he figures uoue of them — an omission shaied in 

 common by all authors, without a single exception, who have adopted 

 this view. 



As will presently be evident, they have never been 

 figured, because they have no existence in fact. 



Opposed to Waldeyer's conclusion, though hardly ever referred 

 to in text-books, is the categorical statement of M. Nussbaum ('81), 

 that the germ-cells of Amphibia arise from no portion of the epi- 

 thelium of the body-cavity, but that, on the contrary, they are direct 

 products of the segmentation. Nussbaum's publication dates from 

 1881, and a quite similar view of the origin of the germ-cells is 

 claimed by Goette as having been stated by himself some years pre- 

 viously ('75). 



In recent years Eigenmann ('92) has traced the origin of the 

 germ-cells in a Teleost (Cymaiog aster = Micrometrus aggregatus) to 

 a very early period : indeed, he writes : "the sex-cells are segregated 

 very early, before any protovertebrae are formed. Judging by 

 their size they are segmentation-cells of the fifth generation or there- 

 abouts". 



Quite recently Eigenmann's results, so far as a very early origin 

 of the germ-cells and their primitive independence of the peritoneum, 

 or, indeed, of the mesoderm, have been confirmed by Wheeler ('99) 

 in the lamprey {Petromyzon planeri). 



The remaining view to be noticed is originally due to Ruckert 

 ('88). According to him the gonads are derived from a certain seg- 

 mented portion of the mesoderm, termed by him the "gono-nephro- 

 tome". They are thus segmental in their first origin. As to whence 

 the germ-cells themselves come, whether from cleavage-products of 

 the egg, or from peritoneal cells of the mesoblast, RtiCKERT and those 

 who have adopted his conclusions are silent. Rückert's original idea, 

 which was apparently founded in fact, has been altered in various 

 ways by subsequent writers. How far the author himself is in agree- 

 ment with these amendments it is difficult to say. Nor would it be 

 fair to this able embryologist to make him responsible for more than 

 the broad statement, that the greater part ^) of the foundation of the 

 "sexual gland" appears in the ventral region of the segmented meso- 

 blast ; and that, since this latter embraces the foundation of the urino- 

 genital system, one may speak of it as the "gono-nephrotome". 



1) Not the whole, as Ruckert distinctly states ('88, p. 272). 



41* 



