EVIDENCES FROM EMBRYOLOGY 165 
“Tn birds, the yolk of an egg is really a single ovum, distended to 
an enormous size by food material. The sperm cell is very much 
smaller and can be seen well only with a high-power microscope. It is 
something like a tadpole in shape, having a small cell body, containing 
a little nucleus, and attached to this a long, whiplike process which 
beats rapidly while the cell is alive, enabling it to seek out and unite 
with the large passive egg in the act of fertilization. Enormous num- 
bers of sperm cells are produced by the male, but only one takes part 
in fertilization. After the first has penetrated the membrane of an 
egg cell, a change takes place in the latter which prevents the entrance 
of others. 
~ “The sperm activates certain formerly inert substances in the egg 
and the new combination cell (the zygote) starts almost at once to 
produce a new individual.” 
OUTLINE OF ANIMAL DEVELOPMENT'’ 
“D. S. JORDAN AND V. L. KELLOGG 
The embryonic development is from the beginning up to a certain 
point practically alike, looked at in its larger aspect, for all the many- 
celled animals. That is, there are certain principal or constant 
characteristics of the beginning development which are present in the 
development of all many-celled animals. The first stage or phenome- 
non of development is the simple fission of the germ cell into halves 
(Fig. 35, 5). These two daughter cells next divide so that there are 
four cells (c); each of these divides, and this division is repeated until 
a greater or lesser number (varying with the various species or groups 
of animals) of cells is produced. These cells may not all be of the same 
size, but in many cases they are, no structural differentiation whatever 
being apparent among them. 
The phenomenon of repeated division of the germ cell is called 
cleavage, and this cleavage is the first stage of development in the 
case of all many-celled animals. The germ or embryo in some animals 
consists now of a mass of few or many undifferentiated primitive cells 
lying together and usually forming a sphere (Fig. 35, ¢), or perhaps 
separated and scattered through the food yolk of the egg. The next 
stage of development is this: the cleavage cells arrange themselves so 
as to form a usually hollow sphere or ball, the cells lying side by side to 
t From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copyright 
1907). Used by special permission of the publishers, D. Appleton & Company. 
