THE KINSHIP OF LIFE. 390 
a larger amount of ‘food yolk’ stored up to supply the 
wants of the growing embryo till the time comes when 
it shall be able to shift for itself. Protoplasm is active, 
‘food yolk’ passive, and the relative amounts of these 
two and the positions which they occupy in the egg 
affect, in a purely mechanical manner, the segmentation, 
and interfere with or destroy its typical regularity. In 
the egg of the common hen this ‘food yolk’ forms 
almost the whole of the yolk, the really important pro- 
toplasm occurring only in the lighter yellow spot, which 
is always uppermost in the egg. Taking it for granted 
that this amount of food yolk influences the character 
of the early stages of development (a point easily proved 
by the embryologists), let us consider a special case in 
which conclusions drawn from development have re- 
ceived later confirmation from other sources. 
“In the mammals the eggs are very small and con- 
sist of pure protoplasm, food yolk being entirely absent. 
Indeed, nourished by the mother, as the 
young of most of these forms are, no 
store of food yolk is necessary. Hence, 
on @ priori grounds, one would say that the segmenta- 
tion of the mammalian egg would be regular in its char- 
acter. When, however, naturalists came to study the 
development of the mammalian egg, it was found that 
in its early stages it presented (in eggs without food 
yolk) some-astonishing peculiarities. How to explain 
these peculiarities was a problem. If, however, it were 
assumed that the mammals have descended from forms 
with larger eggs, and that in the course of evolution 
they have lost the yolk but had retained the tendencies 
of development, the explanation were easy. This ex- 
planation, however, seemed very improbable, for it had 
been held, on grounds of structure, that the mammals 
must have descended from the batrachia, a group con- 
The egg of the 
mammal. 
