of impregnation is nothing more than a sudden GROWTH, in which the
originally simple cell doubles its volume, and is thus prepared for
reproduction (cell-division). Afterwards slight differences are seen
in the size of the copulating cells; though the smaller ones still
have the same shape as the larger ones. It is only when the difference
in size is very pronounced that a notable difference in shape is
found: the sprightly sperm-cell changes more in shape and the ovum in
size.

Quite in harmony with this new conception of the EQUIVALENCE OF THE
TWO GONADS, or the equal physiological importance of the male and
female sex-cells and their equal share in the process of heredity, is
the important fact established by Hertwig (1875), that in normal
impregnation only one single spermatozoon copulates with one ovum; the
membrane which is raised on the surface of the yelk immediately after
one sperm-cell has penetrated (Figure 1.25 C) prevents any others from
entering. All the rivals of the fortunate penetrator are excluded, and
die without. But if the ovum passes into a morbid state, if it is made
stiff by a lowering of its temperature or stupefied with narcotics
(chloroform, morphia, nicotine, etc.), two or more spermatozoa may
penetrate into its yelk-body. We then witness polyspermism. The more
Hertwig chloroformed the ovum, the more spermatozoa were able to bore
their way into its unconscious body.

(FIGURE 1.28. Stem-cell of a rabbit, magnified 200 times. In the
centre of the granular protoplasm of the fertilised ovum (d) is seen
the little, bright stem-nucleus, z is the ovolemma, with a mucous
membrane (h). s are dead spermatozoa.)

These remarkable facts of impregnation are also of the greatest