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. By 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 filaments al- 

 ready noticed: these are the rudiments of the vertebras, 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.! 



The appearance of the elementary filaments of the spinal 

 cord is soon followed by the development of 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 consti- 

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



* The plicae, primitivae of Pander; the laminae dorsoles of Baer. See a 

 paper on embryology by Dr. Allen Thomson, in the Eclin. New Phil. Journal 

 for 1830 and 1831. 



f These rings have, by speculative physiologists, been supposed to be 

 analogous to those which form the skeleton of the Annelida. 



