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behaves as a plastic, semi-fluid mass. The change in shape of the 
whole mass, which goes on continuously from the time of appearance 
of the dorsal lip until the blastopore is small and the segmentation 
cavity obliterated, shows that during gastrulation the yolk is far from 
inactive. An examination of sections through successive stages shows 
plainly that as the dorsal and ventral (and all) parts of the blasto- 
pore lip advance toward the lower pole, the diameter of the yolk 
plug becomes smaller and smaller, the dorsal and ventral lips con- 
tinuing to press upon it. Thus while it is conceivable that the cells, 
occupying the centre and greater part of the exposed yolk area, may 
remain passive, it is plain that the peripheral yolk cells must con- 
stantly be passing round the edge of the yolk plug, and therefore 
constantly undergoing a change of front. Whereas at one time they 
face downward (in the normal position of the egg), later having 
passed round the edge, the same cells will face toward the dorsal 
or ventral or one of the lateral surfaces of the embryo. When there- 
fore I speak of the yolk area being overgrown by the dorsal and 
ventral lips, I do not intend to imply that the exposed area remains 
altogether passive, while the blastopore lip becomes raised up and 
overgrows the motionless yolk. My idea is rather that as the blasto- 
pore lip from all quarters approaches the lower pole, it presses upon 
the yolk plug, the peripheral material of the latter constantly passing, 
as if it were squeezed, into the interior, while the more central portion 
of the exposed surface of the plug remains quite at rest. The pro- 
gress of the several parts of the blastopore lip toward the lower pole 
is consequently marked not only by the continuous involution of ecto- 
derm cells round the margin of the lip, but also by the continuous 
involution of yolk cells round the margin of the yolk plug. 
It would seem that as the yolk cells thus come to lie nearer, and 
eventually pass round the edge of the yolk plug, they might undergo 
an observable change of shape. As regards this point, I have made 
some though scanty observations on the living embryo. The cells in 
the central part of the yolk area are polygonal, cell-axes not far from 
equal in most cases, the cells forming an epithelium-like mosaic. 
The marginal yolk cells some three tiers deep, in the region of the 
recently formed ventral lip, are on the contrary irregular and present 
a very different appearance from the more centrally situated cells. 
The marginal cells are commonly much longer than wide, the long 
axis of the cell making any angle with the blastopore lip. The corners 
of the cells too are frequently rounded, pigmented intercellular de- 
pressions being present. The distinctive shape of these cells prob- 
