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



appears only at a comparatively late stage, and by a change so 

 abrupt and striking as to possess an absolutely dramatic 

 interest 1 )." And so on. I refrain from further quotation, because 

 Wilson's work contains no real solution of the problem. 



To my mind the solution was lacking, because, on the one hand, 

 it was not recognised, that the mode of development was by means 

 of an alternation of generations; and, on the other, the history of the 

 primary germ-cells in Nereis was, and is, unknown. 



If the reader will compare Wilson's statements with the course 

 of development depicted in my diagram not forgetting, I trust, that 

 the latter is a diagram, and nothing more - - the meaning of the spiral 

 cleavage and of the sudden and abrupt change, of which Wilson 

 speaks, may become apparent. 



The apical mode of growth, so characteristic of the early for- 

 mation of the asexual generation in both plants and animals, and 

 which is retained for the whole life-span of the sporophyte of plants, 

 might also be described as spiral. Indeed, it is so regarded and 

 described by botanists. Then with the cutting off' of the connection 

 between the primitive germ-cell and the asexual generation or phoro- 

 zooii we witness the practical end 2 ) of the spiral mode of cleavage, 

 and the commencement of the bilateral period. With this the formation 

 of the primary germ-cells is connected, following the genesis of these 

 a start is made in the building up of the embryo. 



In this way my diagram gives a general interpretation of Wil- 

 son's finds, not to mention those of other observers. 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 fit into an antithetic alternation 

 of generations, 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 



1) Spaced by me. 



2) The practical end but not the actual termination; for, as Wilson 

 points out (p. 393), n it is only in the peculiar changes involved in the for- 

 mation a larval organ, the prototroch, that the spiral form of division overlaps 

 the bilateral period". 



