CHAPTER XIII. 



DEVELOPMENT OF THE NERVOUS SYSTEM, ORGANS OF 

 SENSE, SKELETON, AND LIMBS. 



fTIHE first trace of the cerebro-spinal axis in the embryo consists of 

 J- the two longitudinal folds of the external blastodermic layer, 

 including between them the median furrow, known as the "medullary 

 groove " (page 630). When these folds have united on the median 

 line, converting the groove into a canal, the cavity thus produced 

 assumes the name of the "medullary canal," within and around which 

 the central nervous system is formed. 



FIG. 218. 



FIG. 217. 



FORMATION OF THE 

 CEREBRO - SPINAL 

 Axis. a, b. Spinal 

 cord. c. Cephalic ex- 

 tremity, d. Caudal 

 extremity. 



FCETAL PIG, 1% centimetre long, 

 showing the condition of the 

 brain and spinal cord. 1. Hemi- 

 spheres. 2. Tubercula quadci- 

 gemina. 3. Cerebellum. 4. Me- 

 dulla oblongata. 



FORMATION OF THE CERK- 

 BRO-SPINAL Axis. 1. Vesi- 

 cle of the hemispheres. 2. 

 Vesicle of the tubercula 

 quadrigemina. 3. Vesicle 

 of the medulla oblougata. 



Its mode of formation is by the growth of nervous matter from 

 the inner surface of the medullary canal. The cerebro-spinal axis, 

 accordingly, is at first a hollow longitudinal cord, varying in size in 

 different regions (Fig. 217). Its anterior part expands into a bulbous 

 enlargement corresponding to the brain. Its posterior part, which rep- 

 resents the spinal cord, is nearly cylindrical, terminating at its caudal 

 extremity by a pointed enlargement. 



The next change is a division of the anterior bulbous enlargement 

 into three secondary compartments, partially separated from each other 



667 



