Jax. 1902.] BOTANICAL SOCIETY OF EDINBURGH 139 



And thus the phenomena observed in the development 

 of Nereis are seen to be due to an antithetic alternation 

 of generations, where the asexual generation arises in a 

 spiral or apical manner, where the sexual generation is 

 characterised by a bilateral mode of formation, and, lastly, 

 where one may predict the formation of a primitive 

 germ-cell, and of primary germ-cells from this, between the 

 two generations — that is to say, prior to the development of 

 the sexual generation. 



In the course of more than twelve years spent in the 

 attempt to elucidate the mode of Metazoan development, 

 at various times many things have seemed inexplicable ; 

 but wherever their history has been discovered, they have 

 been found to tit into an antithetic alternation of genera- 

 tions, and into nothing else. 



If Wilson's finds be not based in such an alternation, 

 but be in connection with a " direct " mode of development, 

 they seem to me to include facts which will never be 

 explicable, for such a roundabout kind of development can 

 hardly be termed " direct." Or shall we " explain " and 

 describe them as the development of the Scyphozoa is 

 explained and described in almost all the current text- 

 books, by the omission of any reference to the main portion 

 of the asexual generation, the stolon, discovered by Sars ? ^ 



Such a course may simplify matters, but it hardly makes 

 for the discovery of the facts of nature ! 



Eeverting to the diagram of the life-cycle of the skate, 

 I consider it to be possible at present only by comparison 

 and induction to show the fate of the cells to the left of 

 the " germinal track " as far as U.K.Z., the primitive germ- 

 cell. The comparison with other cases only goes to show 

 its correctness, and, I am convinced, the number of such 

 will increase in the proportion as the study of cell-lineage, 

 so ably established by Whitman, Mark, and E. B. Wilson, 

 replaces the pursuit of the three sacred layers of embryo- 

 logists. 



Up to the point U.K.Z. of my diagram the germinal 

 track in Weismann's sense lies apparently in the larva. 

 It may be objected that in making this substitution the 



1 M. Sars. " Ueber die Entwicklung der Medusa auriia und Cyanea 

 ca2nllata" "Arch. f. Naturgesch," vol. 7, 1841. 



