cuss the comparison.

-Development_

_The Development of the Frog_

Section 1. We have now to consider how the body of the frog is built
up out of the egg cell, but previously to doing so we must revert to the
reproductive organs of our type.

Section 2. In the testes of the male is found an intricate network of
tubuli, the lining of which is, of course, an epithelium. The cells of this
epithelium have their internal borders differentiated into spermatozoa,
which, at a subsequent stage, are liberated. A spermatozoon (Figure
3, Sheet 13, sp.) is a rod-shaped cell containing a nucleus; in fact,
consisting chiefly of nucleus, with a tail, the flagellum, which is
vibratile, and forces the spermatozoon, forward by its lashing. The
spermatozoa float in a fluid which is the joint product of the testes,
anterior part of the kidney, and perhaps the prostate glands.

Section 3. In the ovary, the ova are formed, and grow to a
considerable size. They are nucleated cells, the nucleus going by
the special name of the germinal vesicle and the nucleolus the
germinal spot. The ova prey upon the adjacent cells as they develop.
The protoplasm of the ovum, except at that part of the surface where
the germinal vesicle lies, is packed with a great amount of food
material, the yolk granules. This yolk is non-living inert matter. An
ovum such as this, in which the protoplasm is concentrated towards
one pole, is called telolecithal.

Section 4. After the ovum has finished its growth, and elaborated the
yolk within itself, a peculiar change occurs in the small area free from
yolk-- the animal pole, in which the germinal vesicle lies. This
germinal vesicle divides, and one moiety is budded off from the ovum.
The ovum has, in fact, undergone cell division into a very large cell
containing most of its substance, and a small protoplasmic pimple
surrounding half of its nucleus. The disproportion is so great between
the two cells, that the phenomenon does not at first suggest the idea
of cell division, and it is usually described as the extrusion of the
first polar body. There follows a second and similar small cell,
behind the first, the second polar body. Since the nucleus of the
ovum has divided twice, it is evident that the nucleus remaining now in
the ovum is a quarter of the original nucleus. Very little protoplasm is
given off with the polar bodies; they play no further part in
development, but simply drop off and disappear. No