138 THANSACTIONS AND PROCEEDINGS OF THE [Sess. Lxvi. 



appearance of Ijilateral symmetry in the cleavage. The 

 meaning of the bilateral cleavages in themselves is 

 perfectly obvious. They are the forerunners of the 

 bilateral arrangement of parts in the adult ; and, as 

 such, their explanation belongs to the general problem 

 of bilateral symmetry, which need not be considered 

 here. The all-important point is that the bilaterality 

 does not appear at the beginning of development. It 

 appears only at a comparatively late stage, and by a 

 change so abrupt and striking as to possess an absolutely 

 dramatic interest." And so on. I refrain from further 

 quotation, because Wilson's work contains no real solution 

 of the problem. 



To n)y mind the solution was lacking, because, on the 

 one hand, it was not recognised that the mode of develop- 

 ment 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 formation 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 genera- 

 tion or phorozoon we witness the practical end ^ 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 unfolding of the embryo. 



In this way my diagram gives a general interpretation 



of Wilson's finds, not to mention those of other observers. 



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

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

 formation a larval organ, the prototroch, that the spiral form of 

 division overlaps the bilateral period." 



