REPRODUCTION. 63 
or later, and division of cell contents results. In some cases 
(septic monads) the resulting cell may burst and give rise to a 
shower of animal dust visible only by the highest powers of the 
microscope, each particle of which proves to be the nucleus 
from which a future individual arises. 
The study of reproduction thus establishes the conception of 
a unity of method throughout the animal and, it may be added, . 
the vegetable kingdom, for reproduction in plants is in all main 
points parallel to that process in animals, 
But why that costly loss of protoplasm by polar globules ? 
For the present we shall only say that it appears necessary to 
prevent parthenogenesis; or at least to balance the share which 
the male and female elements take in the work of producing a 
new creature. It is to be remembered that both the male and 
female lose much in the process—blood, nervous energy, etc., in 
the case of the female, while the male furnishes a thousand-fold 
more cells than are used. But the period when organisms are 
best fitted for reproduction is that during which they are also 
most vigorous, and can best afford the superfluous drain on 
their energies. 
SEGMENTATION AND SUBSEQUENT CHANGES. 
After the changes described in the last chapter a new epoch 
in the biological history of the ovum—now the odsperm (or fer- 
tilized egg)—begins. A very distinct nucleus (segmentation 
nucleus) again appears, and the cell assumes a circular outline. 
The segmentation or division of the ovum into usually fairly 
equal parts now commences. This process can be best watched 
in the microscopic transparent ova of aquatic animals which 
undergo perfect development up to a certain advanced stage 
in the ordinary water of the ocean, river, lake, etc.,in which 
the adult lives. 
Segmentation among invertebrates will be first studied, and 
for this purpose an ovum in which the changes are of a direct 
and uncomplicated nature will be chosen. 
The following figures and descriptions apply to a mollusk 
(Elysia viridis). We distinguish in ova resting stages and stages 
of activity. It is not; however, to be supposed that absolute 
rest ever characterizes any living form, or that nothing is tran- 
spiring because all seems quiet in these little biological worlds ; 
for we have already seen reason for believing that-life and in- 
cessant molecular activity are inseparable. It may be that, in 
