910 F. M, BALFOUR. 
larger portion of the ovum, and a distinction is frequently 
made between the germinal disc and the yolk. 
This distinction is, however, apt to lead toa misconception 
of the true nature of the egg. There are strong grounds 
for believing that the so-called yolk, equally with the ger- 
minal disc, is composed of an active protoplasmic basis 
endowed with the power of growth, in which passive yolk 
spherules are embedded ; but that the part ordinarily called 
the yolk contains such a preponderating amount of yolk 
spherules that the active basis escapes detection, and does 
not exhibit the same power of growth as the germinal disc. 
With the exception of mammals, whose development re- 
quires to be more completely investigated, Amphioxus is as far 
as we know the only vertebrate whose ovum does not contain 
a large amount of food material. 
In none of these (vertebrate) yolk-containing ova is the food 
material distributed uniformly. It is always concentrated 
much more at one pole than at the other, and the pole at 
which it is most concentrated may be conveniently called the 
lower pole of the egg. 
In eggs in which the distribution of food material is not 
uniform segmentation does not take place with equal 
rapidity through all parts of the egg, but its rapidity is, 
roughly speaking, inversely proportional to the quantity of 
food material. 
When the quantity of food material in a part of the egg 
becomes very great, segmentation does not occur at all; and 
even in those cases where the quantity of food yolk is not 
too great to prevent segmentation the resulting segmenta- 
tion spheres are much larger than where the yolk-granules 
are more sparsely scattered. 
The frog is the vertebrate whose development comes 
nearest to that of Amphioxus, as far as the points we are at 
present considering are concerned. But it will perhaps facili- 
tate the understanding of their relations shortly to explain the 
diagrammatic sections which I have given of an animal 
supposed to be intermediate in its development between the 
Frog and Amphioxus. Plate X, fig. B 1, represents a longi- 
tudinal section of this hypothetical egg at the close of seg- 
mentation. ‘The lower pole, coloured green, represents the 
part containing more yolk material, and the upper pole, 
coloured yellow, that with less yolk. Owing to the presence 
of this yolk the lower pole even at the close of segmentation 
is composed of cells of a different character to those of the 
upper pole. In this respect this egg can already be distin- 
guished from that of Amphioxus, in which no such difference 
