ORGANIC DEVELOPEMENT. 543 



and which soon form, for this purpose, a series of 

 rings embracing that organ.* 



The appearance of the elementary filaments of 

 the spinal cord is soon followed by the develope- 

 mentof its upper or anterior extremity, from which 

 there arise three vesicles, each forming white 

 tubercles; these are the foundations of the future 

 brain. The tubercles are first arranged in pairs 

 and in a longitudinal series, like those we have seen 

 constituting the permanent form of the brain in the 

 inferior fishes : but, in birds, they are soon folded 

 together into a rounded mass ; while, in the mean 

 time, the two filaments of the spinal cord have 

 approached each other, and united into a single 

 column, the form which they ever after retain. Even 

 at this early period the rudiments of the organs of 

 the higher senses, (first of the eye, and next of the 

 labyrinth of the ear,) make their appearance ; but, 

 on the other hand, those of the legs and wings do 

 not show themselves until the brain has acquired 

 greater solidity and developement. The nerves 

 which are to connect these organs of sensation and 

 of motion with the spinal cord and brain are formed 

 afterwards, and are successively united to the 

 nervous centres. 



Although the plan of the future edifice has thus 

 been sketched, and its foundations laid in the 

 homogeneous jelly, by the simpler efforts of the 

 vital powers, the elevation of the vast superstructure 

 demands the aid of other machinery, fitted to col- 

 lect and distribute the requisite materials. Here, 

 then, we might, perhaps, expect to meet with a 



* These rings liave, by speculative pliysiologists, been supposed to 

 be analogous to those which form the skeleton of the Annelida. 



