398 GENERATION. 



eloDgated and convoluted, until they are fully developed only 

 a short time before birth. 1 



Development of the Nervous System. 



"We have seen, in studying the development of the spinal 

 column, how the dorsal, or medullary plates close over the 

 groove for the neural canal. In the interior of this canal, 

 the cerebro-spinal axis is developed, by cells, which gradually 

 encroach upon its caliber, until we have remaining only the 

 small central canal of the spinal cord, communicating with 

 the ventricles of the brain. As the nervous tissue is devel- 

 oped in the interior of this canal, there is a separation of the 

 histological elements at the surface, to form the membranes. 

 The dura mater and the pia mater are formed first, appear- 

 ing at about the end of the second month, while the arach- 

 noid is not distinct until the fifth month. 2 The nerves, ac- 

 cording to Longet, are not produced as prolongations from 

 the cord into the various tissues, nor do they extend from 

 the tissues to the cord ; but they are developed, in each tis- 

 sue, by a separation of histological elements from the cells 

 of which the parts are originally constituted, which at first 

 appear to be identical in their morphological characters. 3 

 The nerves of the sympathetic system are developed in the 

 same way. 



The mode of development of the spinal cord is thus suffi- 

 ciently simple ; but, with the growth of the ernbryon, we ob- 

 serve dilatations at the superior and at the inferior extremi- 

 ties of the neural canal. The cord is uniform in size in the 

 dorsal region, marked only by the regular enlargements at 

 the sites of origin of the spinal nerves ; but we soon observe 

 an ovoid dilatation below, which forms the lumbar enlarge- 

 ment, from which the nerves are given off to the inferior ex- 



1 BIESIADECKI, in STRICKER, Manual of Human and Comparative Histology, 

 The New Sydenham Society, London, 1872, vol. ii., p. 240, 



3 LONGET, Traite de physiologic, Paris, 1869, tome iii., p. 893. 

 Loc. cit. 



