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SCIENCE 



[N. S. Vol. XXXVI. No. 923 



cells or of minute portions of living sub- 

 stance which have not yet attained to that 

 dignity. We most of us speak and think of 

 life as it occurs in ourselves and other ani- 

 mals with which we are familiar ; and as we 

 find it in the plants around us. We recog- 

 nize it in these by the possession of certain 

 properties — movement, nutrition, growth, 

 and reproduction. We are not aware by 

 intuition, nor can we ascertain without the 

 employment of the miscroscope, that we 

 and all the higher living beings, whether 

 animal or vegetable, are entirely formed of 

 aggregates of nucleated cells, each micro- 

 scopic and each possessing its own life. 

 Nor could we suspect by intuition that 

 what we term our life is not a single indi- 

 visible property, capable of being blown 

 out with a puff like the flame of a candle; 

 but is the aggregate of the lives of many 

 millions of living cells of which the body is 

 composed. It is but a short while ago that 

 this cell-constitution was discovered: it 

 occurred within the lifetime, even within 

 the memory, of some who are still with us. 

 What a marvellous distance we have 

 travelled since then in the path of knowl- 

 edge of living organisms! The strides 

 which were made in the advance of the 

 mechanical sciences during the nineteenth 

 century, which is generally considered to 

 mark that century as an age of unexampled 

 progress, are as nothing in comparison 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 accumu- 

 lated within the same period. And not the 

 least remarkable of these facts is the dis- 

 covery of the cell-structure of plants and 

 animals ! 



Let us consider how cell-aggregates came 

 to be evolved from organisms consisting of 

 single cells. Two methods are possible — 

 viz. : (1) the adhesion of a number of orig- 



inally separate individuals; (2) the sub- 

 division of a single individual without the 

 products of its subdivision breaking loose 

 from one another. No doubt this 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 first perfectly similar in structure and 

 in function; there was no subdivision of 

 labor. All would take part in effecting 

 locomotion; all would receive stimuli from 

 outside; all would take in and digest nu- 

 trient matter, which would then be passed 

 into the cavity of the sphere to serve as a 

 common store of nourishment. Such or- 

 ganisms are still 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 be- 

 came correspondingly altered in shape. 

 With this change in structure differentia- 

 tion of function between the cells cover- 

 ing the outside and those lining the inside 

 of the cup made its appearance. Those on 

 the outside subserved locomotor functions 

 and received and transmitted 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 nourish- 

 ment of all the cells composing the organ- 

 ism. The further course of evolution pro- 

 duced many changes of form and ever-in- 

 creasing complexity of the cavity thus pro- 



