REPRODUCTION. 53 
the access of any male cell (parthenogenesis) ; such, however, is 
almost never the exclusive method of increase for any group of 
animals, and is to be regarded as a retention of a more ancient: 
method, or perhaps rather a reversion to a past biological con- 
dition. No instance of complete parthenogenesis is known 
among vertebrates, although in birds partial development of the 
egg may take place independently of the influence of the male 
sex. The best examples of parthenogenesis are to be found 
among insects and crustaceans. 
It is to be remembered that, while the cells which form the 
tissues of the body of an animal have become specialized to 
discharge one particular function, they have not wholly lost 
all others; they do not remain characteristic amceboids, as we 
may term cells closely resembling Amceba in behavior, nor do 
they wholly forsake their ancestral habits. They all retain the 
power of reproduction by division, especially when young and 
most vigorous; for tissues grow chiefly by the production of 
new cells rather than the enlargement of already mature ones. 
Cells wear out and must be replaced, which is effected by the 
processes already described for Amoeba and similar forms. 
Moreover, there is retained in the blood of animals an army of 
cells, true amceboids, ever ready to hasten to repair tissues lost 
by injury. These are true remnants of an embryonic condition; 
for at one period all the cells of the organism were of this 
undifferentiated, plastic character. But the cell (ovwm) from 
which the individual in its entirety and with all its complexity 
arises mostly by the union with another cell (spermatozoén), 
must be considered as one that has remained unspecialized 
and retained, and perhaps increased its reproductive functions. 
They certainly have become more complex. The germ-cell 
may be considered unspecialized as regards other functions, but 
highly specialized in the one direction of exceedingly great 
capacity for growth and complex division, if we take into ac- 
count the whole chain of results; though in considering this it 
must be borne in mind that after a certain stage of division 
each individual cell repeats its ancestral history again; that is 
to say, it divides and gives rise to cells which progress in turn 
as well as multiply. From another point of view the ovum is 
a marvelous storehouse of energy, latent or potential, of course, 
but under proper conditions liberated in varied and unexpected 
forms of force. It is asort of storehouse of biological energy 
in the most concentrated form, the liberation of which in sim- 
pler forms gives rise to that complicated chain of events which 
