t only in the frog's
ovum, but in all vertebrata, two polar bodies are given off in this way
before the sexual process occurs. Their exact meaning has been
widely discussed. It is fairly evident that some material is removed
from the nucleus, which would be detrimental to further developments,
and the point debated is what is the precise nature of this excreted
material. This burning question we can scarcely deal with here.

Section 5. But here we may point out that in all cells the function of
the nucleus appears to be to determine growth and division. It is the
centre of directive energy in the cell.

Section 6. Fertilization is effected by a spermatozoon meeting with
the ovum. It fuses with it, its nucleus becoming the male pro-nucleus.
This and the female pro-nucleus, left after the extrusion of the polar
cells, move towards each other, and unite to form the first
segmentation nucleus.

Section 7. The ovum next begins to divide. A furrow cutting deeper
and deeper divides it into two; another follows at right angles to this,
making the two four, and another equatorial furrow cuts off the animal
pole from the yolk or vegetative pole. (See Sheet 22, Figures 1, 2,
and 3.) And so segmentation (= cleavage) proceeds, and, at last, a
hollow sphere, the blastosphere (Figure 4) is formed, with a
segmentation cavity (s.c.). But, because of the presence of the yolk
at the vegetative pole of ovum, and of the mechanical resistance it
offers to the force of segmentation, the protoplasm there is not nearly
so finely divided-- the cells, that is to say, are much larger than at
the animal pole. The blastosphere of the frog is like what the
blastosphere of amphioxus would be, if the future hypoblast cells
were enormously larger through their protoplasm being diluted with
yolk.

Section 8. The next phase of development has an equally curious
resemblance to and difference from what occurs in the case of the ova
of animals which do not contain yolk. In such types (e.g., amphioxus)
a part of the blastosphere wall is tucked into the rest, and a gastrula
formed by this process of invagination. In the frog (Figure 5) there is a
tucking-in, but the part that should lie within the gastrula, the
yolk-containing cells, are far larger than the epiblast (ep.) which
should, form the outer layer of cells. Hence the epiblast can only by
continual growth accommodate what it must embrace, and the
process of tucking-in is accompanied by one