52 ANIMAL PHYSIOLOGY. 
logical behavior of tissues and organs. It may be said to be 
quite as important that the ancestral history of the cells of an 
organism be known as the history of the units composing a 
community. A, B, and C can be much better understood if 
we know something alike of the history of their race, their an- 
cestors, and their own past; so is it with the study of any indi- 
vidual, animal, or group of animals or plants. Accordingly, 
embryology, or the history of the origin and development of 
tissues and organs, will occupy a prominent place in the va- 
rious chapters of this work. The student will, therefore, at 
the outset be furnished with a general account of the subject, 
while many details and applications of principles will be left 
for the chapters that treat of the functions of the various organs 
of animals. The more knowledge the student possesses of zo- 
ology the better, while this science will appear in a new light 
under the study of embryology. 
Animals are divisible, according to general structure, into 
Protozoa, or unicellular animals, and Metazoa, or multicellular 
forms—that is, animals composed of cell aggregates, tissues, or 
organs. Among the latter one form of reproduction appears 
for the first time in the animal kingdom, and becomes all but 
universal, though it is not the exclusive method; for, as seen in 
Hydra, both this form of generation and the more primitive 
gemmation occur. It is known as sexual multiplication, which 
usually, though not invariably, involves conjugation of two un- 
like cells which may arise in the same or different individuals. 
That these cells, known as the male and female elements, the 
ovum and the spermatozoén, are not necessarily radically differ- 
ent, is clear from the fact that they may arise in the one individ- 
ual from the same tissue and be mingled together. These cells, 
however, like all others, tell a story of continual progressive 
differentiation corresponding to the advancing evolution of 
higher from lower forms. Thus hermaphroditism, or the coex- 
istence of organs for the production of male and of female cells 
in the same individual, is confined to invertebrates, among 
which it is rather the exception than the rule. Moreover, in 
such hermaphrodite forms the union of cells with greater differ- 
ence in experiences is provided for by the union of different in- 
dividuals, so that commonly:the male cell of one individual 
unites with (fertilizes) the female cell of a different individual. 
It sometimes happens that among the invertebrates the cells 
produced in the female organs of generation possess the power 
of division, and continued development wholly independently of 
