THE PHYSICAL BASIS OF HEREDITY. 169 
in order that they may be able to come together in the 
water in which they are usually set free. The second 
requires that there be furnished a sufficient amount of 
nutritive material for the nourishment of the embryo 
until it arrives at a stage of growth in which it can shift 
for itself. These two necessities have been met by a 
physiological division of labour between the two con- 
jugating cells. The one, the sperm cell, has become 
reduced in size with a corresponding gain in motility, 
the other, the egg cell, has had food yolk stored up in 
it, and its consequent increased size prevents any more 
than a very slight degree of independent movement, if 
any. Different stages of these modifications may be 
met with among unicellular forms, as illustrated above 
in Pandorina, Eudorina, and Volvox, to which might be 
added many others. In Pandorina the conjugating cells 
are of nearly equal size, in Audorina an intermediate 
condition is reached, while in Vo/vox the egg and sperm 
cells are sharply differentiated in size and motility. 
Again, in the first two and their allies a// of the cells are 
at first vegetative and afterward reproductive, while in 
Volvox the definite separation into vegetative or 
somatic, and reproductive or germinal cells makes its 
appearance. 
We arrive then at the conclusion, from the considera- 
tion of these and many other lines of evidence, that the 
germ cells were primitively exactly alike, 
Seon ara and that the differences between them 
e . . . . 
aaa have arisen in the process of differentia- 
germ cells. 
tion along two separate lines. Further- 
more, it is clear that the differences between the two 
sexes, which become strongly characterized in the higher 
vertebrates, are all of a purely secondary nature. 
In their early development the germ cells are indis- 
tinguishable from each other, and both pass through 
