Ja.v. 1902.] BOTANICAL SOCIETY OF EDINBURGH 133 



phenomena of the life-cycles of the ]\Ietazoa 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 stated, from the writings of other embryolo- 

 gists. 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 diagrams of oogenesis and spermatogenesis 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 oogenesis 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.Iv.Z. (the " Urkeimzelle " of German authors), is 

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

 however, from Z to U.K.Z. marks what Weismann terms 

 the " germinal track " (Keimbahn), and the products 

 to the left of it are assumed to be cells of the 

 embryo. As in the skate there is no possibility 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 Ascaris mcyaloccphala, 

 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 primary 

 germ-cells of Ascaris, formed by division of the cell 

 U.K.Z., the one will be seen to form the embryo or 



