Beard, Heredity and the epicycle of the germ-cells. 355 



If the pole-inesoderin-cells be not eggs, they at least arise by the 

 division of the next thing to an egg, a primary germ-cell. 



In Nereis the remaining primary germ-cell D conies to form part 

 of the hypoblast. There is no difficulty about this. Even in the skate 

 many of the primary germ-cells may for a time lie in the hypoblast, 

 but they do not give rise to hypoblastic cells. As Wilson remarks, 

 ,,the ultimate court of appeal lies in the fate of the cells" (loc. 

 cit. 2 p. 41). 



Another apparent difficulty, more especially to the view of the 

 complete similarity and equivalence of the primary germ-cells, would 

 be, that sometimes the embryonic cell may perhaps exceed (?) the pri- 

 mary germ-cells in size. As an instance, that D and M mentioned 

 above may be of different sizes (?). But this very difference in size may 

 serve to explain why some particular primary germ-cell is chosen to 

 form an embryo instead of some other. Position alone cannot always 

 be at the bottom of this. In the skate, for example, the embryo does 

 not invariably begin to arise at one certain spot upon the blastoderm. 

 It may be, that the stimulus afforded by an extra amount of food-yolk 

 may have much to do with the initiation of development. 



Very suggestive and significant in the light of my results in the 

 skate are the following passages from E. B. Wilson's memoir on 

 ,,The Cell-Lineage of .Nereis". Statements equally pregnant with 

 meaning will be found in various parts of Eisig's work on the deve- 

 lopment of Capitella (Mitteil. a. d. Zool. Stat. zu Neapel, V. 13, 

 p. 1292, 1898). 



On page 398 Wilson writes: ,,Transition to the Bilateral Period. 

 As far as the development of the permanent organs is concerned, the 

 transition from the spiral to the bilateral type of development is remark- 

 ably abrupt." 



It may be mentioned, that at the close of the spiral period there 

 are, according to Wilson, 38 blastomeres present. That is to say, 

 the majority of them are products of the fifth cleavage. 



On page 444 he asks ,,what is the significance of the spiral and 

 bilateral forms of cleavage, and where lie the causes that determine 

 the transformation of the one into the other?" Further on he writes: 

 ,,The most striking feature in the cleavage, and the one on which the 

 entire discussion may be made to turn, is the sudden appearance of 

 bilateral 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 1 ). It 



1) Spaced in the original. 



23* 



