Beard, Heredity and the epicj'cle of the germ-cells. 327 



With this recognition it becomes possible to compare together, so 

 as to show their essential similarity, the phenomena of the life-cycles 

 of the Metazoa and Metaphyta. 



In the same way the discovery of the formation of the primary 

 germ-cells and of the epoch of their coming-into-being throw new and 

 unexpected light on the course and nature of heredity. 



These are the chief results of my work on the germ-cells: and, 

 though other and doubtless important finds have been made, the latter 

 sink into insignificance, when placed beside the former. 



Certain parts of the diagram have been adopted, as already sta- 

 ted, from the writings of other embryologists. This, however, has not 

 been done without important modifications, for which the writer is 

 alone responsible. 



Originally towards the close of last year (1900) Boveri's dia- 

 grams of oogeuesis and sperrnatogenesis formed and filled in portions 

 of the life- cycle. Doubts however, arose as to their completeness, and 

 the working out of the probable course of ob'genesis in the skate 

 finally resulted in the modifications here depicted. The first part of 

 the figure, from the zygote Z, formed by the union of egg and sperm, 

 to the primitive germ-cell U. K. Z. (the ,,Urkeimzelle" of German au- 

 thors) is from Boveri's and Weismann's figures. In their diagrams, 

 however, from Z to U. K. Z. marks what Wei s man n terms the ,,ger- 

 minal track" (Keimbahn), and the products to the left of it are as- 

 sumed to be cells of the embryo. As in the skate there is no possi- 

 bility of the existence of any part of the embryo prior to the formation 

 of U. K. Z., it is out of question, that the said cells can be part of 

 this. It is an assumption, that they are parts of the embryo; for in As- 

 caris megalocephala, for instance, the form to which Boveri's identical 

 diagram refers, it has never been established, that directly from the 

 cleavage of the fertilised egg the sexual generation or embryo takes 

 its origin. The later history is here unknown. Indeed, it may be 

 safely predicted, that, when the facts become known, of the two pri- 

 mary germ-cells of Ascaris, formed by division of the cell U. K. Z., 

 the one will be seen to form the embryo or sexual generation, while 

 the other will furnish its sexual products 1 ). 



1) In Ascaris megalocephala it is at least possible, that the primitive 

 germ-cell is separated off at the fourth cleavage instead of at the fifth. The latter 

 cleavage would then divide the primitive germ-cell into two primary germ-cells, 

 of which the one would go to form the embryo and the other would represent 

 the ,,sexual products". If this be the correct interpretation of the conditions 

 in Ascaris - - a point upon which I do not venture to express an opinion - 

 the subsequent division of the cell, regarded by Boveri and others as the 

 primitive germ-cell, would correspond to the formation of secondary germ- 

 cells in Raja, that is, the parent cell would be a primary germ-cell. 



Regarding the life-history of such a Nematode as Ascaris megalocephala, 



