ORGANIC DEVELOPMENT. 423 



to a higher order of endowments, is placed in subordination 

 to a class of functions, of which there exists no trace in ve- 

 getables, namely, those of the nervous system. }ly intently 

 watching the earliest dawn of organic formation, in the trans- 

 parent gelatinous molecule, for example, which, with its 

 three investing pellicles, constitutes the embryo of a bird, 

 (for the eggs of this class of animals best admit of our fol- 

 lowing this interesting series of changes,) the first opaque 

 object discoverable by the eye is a small dark line, called 

 the primitive trace, formed on the surface of the outermost 

 pellicle. Two ridges then arise, one on each side of this 

 dark line;* and by the union of their edges, they soon form 

 a canal, containing a deposite of semi-fluid matter, which, on 

 acquiring greater consistence and opacity, discloses two 

 slender and delicate threads, placed side by side, and parallel 

 to one another, but separated by a certain space. These are 

 the rudiments of the spinal cord, or the central organ of 

 nervous power, on the endowments of which the whole cha- 

 racter of the being to be formed depends. We may next 

 discern a number of parallel equidistant dots, arranged in 

 two rows, one on the outer side of each of the fdaments al- 

 ready noticed: these are the rudiments of the vertebrae, parts 

 which will afterwards be wanted for giving protection to 

 the spinal marrow, and which soon form, for this purpose, 

 a series of rings embracing that organ. t 



The appearance of the elementary filaments of the spinat 

 cord is soon followed by the development of its u])j)cr 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 consti- 

 tuting the permanent form of the brain in the inferior fishes: 



* The plicae primitivx of Pander; the lamina; dorsales of Bucr. Sec a 

 paper on embryolog-y by Dr. Allen Thomson, in the Eclin. New Phil. Journal 

 for 1830 and 1831. 



f These ring's have, by speculative physiologists, been supposed to be 

 analogous to those which form the skeleton of the Annelida. 



