Embryology. 105 



which is said applies also to the case of plants 

 (Protophyta and Metaphyta). 



A Protozoon (like a Protophyton) is a solitary cell, 

 or a " unicellular organism," while a Metazoon (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 ixroo 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 Protozoon is an 

 isolated physiological cell, and, like cells in general, 

 multiplies by means of what Spencer and Hackel 



