Jan. 1902.] BOTANICAL SOCIETY OF EDIXBUIIGH 137 



form part of the hypoblast. There is no difficulty about 

 this. Even in the skate many of the primary germ-cells 

 nlay 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 primary 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 development- 

 of Capitella ("Mitt. a. d. Zool. Stat, zu Xeapel," v. 13, 

 pp. 1-292, 1898). 



On page 393 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 remarkably 

 abrupt." 



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

 period, there are, according to Wilson, thirty-eight blasto- 

 meres 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 strik- 

 ing feature in the cleavage, and the one on which the 

 entire discussion may be made to turn, is the sudden 



TRAXS. EOT. SOC. EDIX. VOL. XXII. K 



