286 EVOLUTION IN BIOLOGY. [LECT. 



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 

 plantse omnes ex eodem communi nutrimento (sive 

 rore seu terrse humore)." 



In all 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-aggregate 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 central space, around which they 

 dispose themselves as a coat or envelope; and thus 

 the raorula becomes a vesicle filled with fluid, the 

 planula. The wall of the planula is next pushed in 

 on one side, or invaginated, whereby it is converted 



