116 PHYLUM PROTOZOA——-THE SIMPLEST ANIMALS. 
. REPRODUCTION OF PROTOZOA 
Growth and reproduction are on a different plane from 
the other functions. Growth occurs when income exceeds 
expenditure, and when constructive or anabolic processes 
are in the ascendant. Reproduction occurs at the limit of 
growth, or sometimes in disadvantageous conditions. 
As it is by cell division that all embryos are formed from the egg, and 
all growth is effected, the beginnings of this process are of much interest. 
(a) Some very simple Protozoa seem to reproduce by what looks like 
the rupture of outlying parts of the cell substance. (6) The production 
of a small bud from a parent cell is not uncommon, and some Rhizo- 
pods (é.g. rcella, Pelomyxa) give off many buds at once. (c) Com- 
moner, however, is the definite and orderly process by which a unit 
divides into two—ordinary cell division. (d) Finally, if many divisions 
occur in rapid succession or contemporaneously, and usually within a 
cyst enclosing the parent cell, z.e. in narrowly limited time and space, 
the result is the formation of a considerable number of small units or 
spores. In the great majority of cases, each result of division is seen 
to include part of the parent nucleus. 
A many-celled animal multiplies in most cases by 
liberating reproductive cells—ova and spermatozoa — 
different from the somatic cells which make up the “ body.” 
A Protozoon multiplies by dividing wholly into daughter 
cells. This difference between Metazoa and Protozoa in 
their modes of multiplication is a consequence of the 
difference between multicellular and unicellular life. Each 
part of a divided Protozoon is able to live on, and will 
itself divide after a time, whereas the liberated spermatozoa 
and ova of a higher animal die unless they unite. 
By sexual reproduction we mean—(a) the liberation of 
special reproductive cells from a “body,” and (4) the 
fertilisation of ova by spermatozoa. As Protozoa have 
no “body”—though the beginnings of one are seen in 
the colonial forms—they cannot be said to exhibit sexual 
reproduction in the first sense (a), yet many of them 
(especially the Sporozoa) give origin by division to special 
reproductive cells. And although many Protozoa can live 
on, dividing and multiplying, for prolonged periods without 
the occurrence of anything like fertilisation, processes 
corresponding to fertilisation are of general occurrence. 
For in many of the Protozoa there occurs at intervals a 
process of “conjugation” in which two individuals unite 
