822 EMBRYOLOGY 



extremities, the superficial muscles of the back, the oblique muscles of 

 the abdomen and the muscles of the face. 



The skin appears at about the beginning of the second month, when 

 it is thin and transparent. At the end of the second month the epi- 

 dermis may be distinguished. The sebaceous follicles are developed at 

 the third month ; and at about the fifth month the surface is covered 

 with their secretion mixed with desquamated epithelium. This cheesy 

 substance is the vernix caseosa. At the third month the nails make 

 their appearance, and the hairs begin to grow at about the fifth month. 

 The sudoriparous glands first appear at about the fifth month, by the 

 formation of flask-like processes of the true skin, which are gradually 

 elongated and convoluted until they are fully developed only a short 

 time before birth. 



DEVELOPMENT OF THE NERVOUS SYSTEM 



It has been 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 devel- 

 oped, by cells which gradually encroach on its calibre, until there 

 remains only the small central canal of the spinal cord, communicating 

 with the ventricles of the brain. As the nervous tissue is developed in 

 the interior of the neural 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, appearing at about the end of the second 

 month ; while the arachnoid is not distinct until the fifth month. The 

 nerves 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 tissue by a separation of histological elements from 

 the cells of which the parts are originally constituted. The nerves of 

 the sympathetic system are developed in the same way. 



The mode of development of the spinal cord is thus sufficiently 

 simple ; but with the growth of the embryo, dilatations are observed at 

 the superior and the inferior extremities of the neural canal. The cord 

 is nearly uniform in size in the dorsal region, marked only by the 

 regular enlargements at the sites of origin of the spinal nerves; but 

 there soon appears an ovoid dilatation below, which forms the lumbar 

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

 tremities, and the brachial enlargement above, where the nerves of the 

 superior extremities take their origin. At the same time there is a 

 more marked dilatation of the canal at its cephalic extremity. Here 

 a single enlargement appears, which is soon divided into three vesicles, 



