EVOLUTION IN BIOLOGY. . 293 



the living body into which it enters. And, so far from 

 the fully developed organism being simply the germ 

 . plus the nutriment which it has absorbed, it is probable 

 that the adult contains neither in form, nor in substance, 

 more than an inappreciable fraction of the constituents 

 of the germ, and that it is almost, if not wholly, made 

 up of assimilated and metamorphosed nutriment. In 

 the great majority of cases, at any rate, the full-grown 

 organism becomes what it is by the absorption of not- 

 living matter, and its conversion into living matter of 

 a specific type. As Harvey says *(Ex. 45), all parts of 

 the body are nourished " ab eodem succo alibili, aliter 

 aliterque cambiato," " ut plants omnes ex eodem com- 

 muni nutrimento (sive rore seu terrse humore)." 



In alX^animals and plants, above the lowest, the germ 

 is a nucleated cell, using that term in its broadest sense ; 

 and the first step in the process of the evolution of the 

 individual is the division of this cell into two or more 

 portions. The process of division is repeated, until the 

 organism, from being unicellular, becomes multicellular. 

 The single cell becomes a cell-aggregate ; and it is to the 

 growth and metamorphosis of the cells of the cell-aggre- 

 gate thus produced, that all the organs and tissues of the 

 adult owe their origin. 



In certain animals belonging to every one of the 

 chief groups into which the Metazoa are divisible, the 

 cells of the cell-aggregate which results from the process 

 of yelk-division, and which is termed a morula, diverge 

 from one another in such a manner as to give rise to a 



