(FIGURE 1.12. Mobile cells from the inflamed eye of a frog (from the
watery fluid of the eye, the humor aqueus). The naked cells creep
freely about, by (like the amoeba or rhizopods) protruding fine
processes from the uncovered protoplasmic body. These bodies vary
continually in number, shape, and size. The nucleus of these amoeboid
lymph-cells ("travelling cells," or planocytes) is invisible, because
concealed by the numbers of fine granules which are scattered in the
protoplasm. (From Frey.))

In many of the lower animals (such as sponges, polyps, and medusae)
the naked ova retain their original simple appearance until
impregnation. But in most animals they at once begin to change; the
change consists partly in the formation of connections with the yelk,
which serve to nourish the ovum, and partly of external membranes for
their protection (the ovolemma, or prochorion). A membrane of this
sort is formed in all the mammals in the course of the embryonic
process. The little globule is surrounded by a thick capsule of
glass-like transparency, the zona pellucida, or ovolemma pellucidum
(Figure 1.14). When we examine it closely under the microscope, we see
very fine radial streaks in it, piercing the zona, which are really
very narrow canals. The human ovum, whether fertilised or not, cannot
be distinguished from that of most of the other mammals. It is nearly
the same everywhere in form, size, and composition. When it is fully
formed, it has a diameter of (on an average) about 1/120 of an inch.
When the mammal ovum has been carefully isolated, and held against the
light on a glass-plate, it may be seen as a fine point even with the
naked eye. The ova of most of the higher mammals are about the same
size. The diameter of the ovum is almost always between 1/250 to 1/125
inch. It has always the same globular shape; the same characteristic
membrane; the same transparent germinal vesicle with its dark germinal