FUNCTION. 203 



function of transferring energy, thus vaguely indicated in 

 these inferior forms, comes afterwards to be the definitely- 

 separated office of a complicated apparatus made up of many 

 parts, each of which has a particular portion of the general 

 duty, need not be described. It is sufficiently manifest that 

 this general function becomes more clearly marked-off from 

 the others, at the same time that it becomes itself parted 

 into subordinate functions. 



In a developing embryo, the functions or more strictly 

 the structures which are to perform them, arise in the same 

 general order. A like primary distinction very early appears 

 between the endoderm and the ectoderm — the part which 

 has the office of accumulating energy, and the part out of 

 which grow those organs that are the great expenders of 

 energy. Between these two there presently arises the meso- 

 derm in which becomes visible the rudiment of that vascular 

 system, which has to fulfil the intermediate duty of trans- 

 ferring energy. Of these three general functions, that of 

 accumulating energy is carried on from the outset : the 

 endoderm, even while yet incompletely differentiated from 

 the ectoderm, absorbs nutritive matters from the subjacent 

 yelk. The transfer of energy is also to some extent effected 

 by the rudimentary vascular system, as soon as its central 

 cavity and attached vessels are sketched out. But the ex- 

 penditure of energy (in the higher animals at least) is not 

 appreciably displayed by those ectodermic and mesodermic 

 structures that are afterwards to be mainly devoted to it: 

 there is no sphere for the actions of these parts. Simi- 



larly with the chief subdivisions of these fundamental func- 

 tions. The distinction first established separates the office 

 of transforming other energy into mechanical motion, from 

 the office of liberating the energy to be so transformed. 

 While in the layer between endoderm and ectoderm are 

 arising the rudiments of the muscular system, there is marked 

 out in the ectoderm the rudiment of the nervous system. 

 This indication of structures which are to share between 



