MORPHOLOGICAL UNITY OF LIVING BEINGS. I59 



have an average diameter of several thousandths of 

 a millimetre — i.e., of several microns. This element, 

 the cell, is a real organ. It is smaller, no doubt, than 

 those described by the ancient anatomists, but it is not 

 less complex. Its complexity is only revealed later. 

 It is an organic unit. Its form varies from one 

 element to another. Its substance is a semi-fluid 

 mass, a mixture of different albuminoids. In the 

 mean value of its dimensions, so carefully measured — 

 exceptis exxipiendis — we have a condition the signi- 

 ficance of which has not yet been discovered, but 

 which may be of great value in the explanation of its 

 peculiar activities. 



Such is the result to which have converged the re- 

 searches of the biologists who have examined plants 

 or the lower animals, as well as of the anatomists who 

 have been more especially occupied with the verte- 

 brates and with man. All their researches have 

 brought them to the same conclusion — the cellular 

 theory. Either living beings are composed of a single 

 cell — as is the case with the microscopic animals called 

 protozoa, and the microscopic vegetables called proto- 

 phytes — or, they are cellular complexes, metazoa or 

 inetapJiytes — that is to say, associations of these 

 microscopic organic units which are called cells. 



The Lazu of the Composition of Organisms. — The 

 law of the composition of organisms was discovered 

 in 1838 by Schleiden and Schwann. From that time 

 up to 1875 it may be said that micrographers have 

 spent their time in examining every organ and every 

 tissue, muscular, glandular, conjunctive, nervous, etc., 

 and in showing that in spite of their varieties of 

 aspect and form, of the complexity of structures due 

 to cohesion and fusion, they all resolve into the com- 



