Embryology. 105 
which is said applies also to the case of plants 
(Protophyta and Metaphyta). 
A Protozoén (like a Protophyton) is a solitary cell, 
or a “unicellular organism,” while a Metazoén (like a 
Metaphyton) is a society of cells, or a “ multicellular 
organism.” Now, it is only in the multicellular 
organisms that there is any observable distinction of 
sex. In all the unicellular organisms the phenomena 
of reproduction appear to be more or less identical 
with those of growth. Nevertheless, as these phe- 
nomena are here in some cases suggestively peculiar, 
I will consider them more in detail. 
A Protozoon is a single corpuscle of protoplasm, 
which in different species of Protozoa varies in size 
from more than one inch to less than jj455 of an inch 
in diameter. In some species there is an enveloping 
cortical substance; in other species no such substance 
can be detected. Again, in most species there is a 
nucleus, while in other species no such differentiation 
of structure has hitherto been observed. Nevertheless, 
from the fact that the nucleus occurs in the majority 
of Protozoa, coupled with the fact that the demon- 
stration of this body is often a matter of extreme 
difficulty, not only in some of the Protozoa where it 
has been but recently detected, but also in the case of 
certain physiological cells elsewhere,—fror these facts 
it is not unreasonable to suppose that all the Protozoa 
possess a nucleus, whether or not it admits of being 
rendered visible by histological methods thus far at 
our disposal. If this is the case, we should be justi- 
fied in saying, as I have said, that a Protozoén is an 
isolated physiological cell, and, like cells in general, 
multiplies by means of what Spencer and Hackel 
