Beard, Heredity and the epicycle of the germ-cells. 357 



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 1 )? 



Such a course may simplify matters, but it hardly makes for the 

 discovery of the facts of Nature. 



Reverting to the diagram of the life-cycle of the skate, I con- 

 sider 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. Wil- 

 son, replaces the pursuit of the three sacred layers of embrylogists. 



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 embryo has been displaced, in 

 order to establish a more or less problematical larva, and that the 

 germinal track is here somatic. The reply to this is, that the cell 

 U. K. Z. and its immediate ancestors never form part of the larva, 

 and that the period 2 ) from Z. to U. K. Z. no matter how long it 

 be, whether four generations or four thousand is marked by a 

 mode of growth and cell- division, conspicuous by absence in other 

 parts of the diagram 3 ). 



This statement requires both elucidation and emphasis. 



The mode of growth of the sporophyte in plants is essentially 

 apical, that is to say, wherever there is an apex there are always 

 one or more apical cells, which by their division give off products 

 towards the centre. 



In the sexual generation of a Metazoon the mode of growth dif- 

 fers in to to from this; for here all the products ultimately undergo 

 differentiation, and embryonic or germ-material, corresponding to apical 

 cells, has no existence. The older embryologists, of the first half of 



1) M. Sars. Ueber die Entwickluug der Medusa aurita und Cyanea 

 capillata. Arch. f. Naturgesch. Vol. 7, 1841. 



2) In the skate this period includes more than five mitoses, probably ten. 



3) Spemann has already compared the mode of origin of the first 

 cleavage products in Nematodes, more especially in Strongylus, to the apical 

 mode of growth in the^sporophyte of a plant. He notes, that the cell along 

 the line Z - - U. K. Z. in my diagram acts as though it were an apical cell of 

 a sporophyte (H. Spemann, Die Entwicklung von Strongylus paradoxus, 

 Zool. Jahrb. Morph. Abteil. V. 8, p. 304, 189495. 



