510 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 1912. 



lives of many millions of living cells of which the body is composed. 

 It is but a short whUe ago that this cell constitution was discovered: 

 it occurred within the lifetime, even within the memory, of some who 

 are stUl with us. What a marvelous distance we have traveled 

 since then in the path of knowledge of living organisms! The strides 

 which were made in the advance of the mechanical sciences during 

 the nineteenth century, which are generally considered to mark that 

 century as an age of unexampled progress, are as nothing in com- 

 parison with those made in the domain of biology, and their interest 

 is entirely dwarfed by that which is aroused by the facts relating to 

 the phenomena of life which have accumulated within the same period. 

 And not the least remarkable of these facts is the discovery of the 

 cell structure of plants and animals. 



EVOLUTION OF THE CELL AGGREGATE. 



Let us consider how cell aggregates came to be evolved from organ- 

 isms consisting of single cells. Two methods are possible, viz: (1) 

 The adhesion of a number of originally separate individuals; (2) the 

 subdivision of a single individual without the products of its sub- 

 division breaking loose from one another. No doubt tliis last is the 

 manner whereby the cell aggregate was originally formed, since it is 

 that by which it is still produced, and we know that the life history 

 of the individual is an epitome of that of the species. Such aggre- 

 gates were in the beginning solid ; the cells in contact with one another 

 and even in continuity; subsequently a space or cavity became formed 

 in the interior of the mass, which was thus converted into a hollow 

 sphere. All the cells of the aggregate were at fu'st perfectly similar 

 in structure and in function; there was no subdivision of labor. All 

 would take part in effecting locomotion; aU would receive stimuli 

 from outside; all would take in and digest nutrient matter, which 

 would then be passed into the cavity of the sphere to serve as a 

 common store of nourishment. Such organisms are stiU found, and 

 constitute the lowest types of Metazoa. Later one part of the hollow 

 sphere became dimpled to form a cup; the cavity of the sphere 

 became correspondingly altered in shape. With this change in struc- 

 ture differentiation of function between the cells covering the outside 

 and those lining the inside of the cup made its appearance. Those 

 on the outside subserved locomotor functions and received and trans- 

 mitted from cell to cell stimuli, physical or chemical, received by the 

 organism; while those on the inside, being freed from such functions, 

 tended to specialize in the direction of the inception and digestion of 

 nutrient material, which, passing from them into the cavity of the 

 invaginated sphere, served for the nourishment of aU the ceUs com- 

 posing the organism. The further course of evolution produced many 



