30 J. GRAHAM KERR. ', 



particular cells based on the finely granular character of the 

 yolk. I should attach much greater weight to the presence 

 of large granules of yolk in cells, for when the yolk is in 

 this form in a developing embryo it seems usually to indicate 

 that it has remained so all through, it being at least very 

 unusual for yolk to be secondarily built up again into large 

 granules during embryonic development. 



The finely granular character of the yolk frequently shown 

 by the roof, as compared with that of the floor of the arch- 

 enteron, I would look upon then as being merely a necessary 

 accompaniment of the active growth of this region asso- 

 ciated with the backgrowtli of the blastopore lip. 



What I have said regarding the unreliability of evidence 

 of the morphological nature of cells from the finely granular 

 character of their yolk contents applies equally well to the 

 presence of black pigment in cells. I believe it to be one of 

 the most general reactions to light stimulus for active but 

 unspecialised cells to have their metabolism so affected as to 

 cause the formation of this particular product.^ Examples 

 are seen in the case of comparatively undifferentiated cells 

 Avandering into a position where they are subjected to light 

 stimulus, e. g. to the surface of the body, or into the vicinity 

 of a special light-collecting organ (e.g. Arthropod eye). 

 Where pigment occurs in the smaller cells of a frog's egg it 

 is, I think, to be correlated simply with the more active 

 metabolism going on in these cells, and it is rather the 

 absence of pigment in special cases which demands explana- 

 tion ; in many cases this may be due to natural selection — as 

 in the case of eggs which are laid in a floating mass of white 

 foam, where their being black would render them extremely 

 conspicuous.^ 



' Which once produced may well be made use of as a protective agent for 

 neighbouring tissues against the harmful influence of light rays. 



' With the criticisms in the foregoing paragraphs are to be associated those 

 on similar lines of Houssay (' Arch. Zool. exp.,' 2nd ser., t. viii) and Saniassa 

 ('Verli. Deutsch. Zool. Gesell.,' Strasbourg, p. 139; also 'Arch. Entw. 

 Mech.,' Bd. ii and Bd. vii). 



