004 THE REPRODUCTIVE FUNCTIONS. 



side of this dark line* ; and by the union of 

 their edges, they soon form a canal, containing a 

 deposit 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 

 character of the being to be formed depends. 

 We may next discern a number of parallel equi- 

 distant dots, arranged in two rows, one on the 

 outer side of each of the filaments already no- 

 ticed : 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. I 



The appearance of the elementary filaments 

 of the spinal cord is soon followed by the deve- 

 lopement of its upper or anterior extremity, from 

 w^hich there arise three vesicles, each forming 

 w^hite tubercles ; these are the foundations of the 

 future brain. The tubercles are first arranged 



* The pliccB primitivcB of Pander ; the laminee dorsales of 

 Baer. See a paper on embryology by Dr. Allen Thomson, in 

 the Edin. New Phil. Journal for 1830 and 1831. 



f These rings have, by speculative physiologists, been sup- 

 posed to be analogous to those which form the skeleton of the 

 Annelida. 



