168 FOOT-NOTES TO EVOLUTION. 
into bundles of minute antherozooids (sixty-four to a 
hundred and twenty-eight). The remaining cells of the 
colony, remain in a vegetative condition, and eventu- 
ally die. In reproduction, one of the antherozooids 
fuses with one of the oospheres, a resting zygote is 
formed from which develops later a newcolony. Thus 
in the Volvox colony we meet with a differentiation into 
somatic or vegetative cells and reproductive cells, a dif- 
ferentiation which persists through all the multicellular 
plants and animals. 
A much larger series of forms might be cited to 
illustrate the phenomena of multiplication among uni- 
cellular organisms, which would show all stages of gra- 
dation in the relative size of the conjugating cells from 
those in which both are of equal size and are equally 
active, to such forms as Volvox, in which a great dif- 
ference in size exists, the larger, the oosphere, being 
non-motile and laden with food material, the smaller, 
the antherozooid, having the cytoplasm reduced to 
a very small amount and being endowed with high 
mobility. 
In multicellular organisms we meet with a continua- 
tion of the same facts. The animal egg is a single cell 
laden with a large amount of food yolk, 
and made up of nucleus and cytoplasm 
as the living elements. For the develop- 
ment of this egg, conjugation with another germ cell, 
derived from a different individual, is necessary. This 
germ cell is the spermatozooid, a minute cell consisting 
of nucleus and centrosome with a small amount of cyto- 
plasm modified primarily into an organ of locomotion, 
the tail. A physiological division of labour is here met 
with which admirably meets two diametrically opposed 
requirements, The one of these demands that the con- 
jugating cells be highly motile, and consequently small, 
Reproduction in 
Metazoa. 
