494 
.T. GRAHAM KERR. 
in a cylindrical slieatli of splanchnic mesoderm dotted with 
scattered chromatophores. From being a spirally coiled 
structure the intestine has therefore now assumed the out- 
ward form of a straight cylinder, only the cavity in its 
interior betraying its once coiled condition. 
III. Buccal Cavity. 
The Dipnoi share with various Amphibians the peculiarity 
that the main part of the buccal cavity arises in place of the 
anterior portion of the yolk-laden enteric rudiment derived 
from the macromeres of the segmented egg, i.e. its wall is in 
great part deidved from a mass of cells which would ordi- 
narily be called endoderm. Those who are sticklers for the 
sanctity of the germ layer theory find it difficult to accept this 
statement, and would rather believe that, although the defini- 
tive buccal cavity comes into existence in place of a “ primitive 
endoderm ” structure, yet its lining is formed by a definite 
ingrowth of ectoderm. Thus the buccal cavity of the forms 
mentioned would be not merely in theoi’y, but as regards its 
actual ontogenetic development a typical stomodaeum. Greil 
in particular, who has investigated the development in C era- 
tod us, appears to have no doubt that actual ingrowth of 
cells from the outside takes place to form the lining of the 
buccal cavity. Personally I see no reason to depart from 
the statement which I made some years ago^ that the main 
part of the buccal lining arises in situ by actual transforma- 
tion of the originally yolk-laden cells. The active metabo- 
lism associated with this process of transformation is hei’e as 
elsewhere accompanied by a breaking up of the yolk granules 
into very fine particles so that they may be more easily 
assimilable, and it is this assumption of a finely yolked and 
richly protoplasmic character that causes the cells to assume 
an ectoderm-like appearance. “ It is,” as I put it in my 
former paper, as if an influence were spreading inwards 
' ‘ Quarterly Journal of Microscopical Science,’ vol. 46, p. 423. 
