THE PRIMITIVE VERTEBRAL CORD. 305 



the primitive vertebrae and the neighbouring parts. It forms 

 the first rudiment ot the individual segments of the verte- 

 bral column, the primitive vertebrae. At a later period these 

 primitive vertebrae become very closely connected with the 

 chorda dorsalis which they surround, and this whole axis- 

 mass then develops into the vertebral column, which is 

 afterwards articulated in so many complex ways. The 

 peripheral parts of the two side-layers, which remain after 

 the separation of the primitive vertebral cord, are hence- 

 forth called the side-plates (lamellae), the term being thus 

 used in its restricted sense. They develop into the two 

 fibrous layers, which have already been mentioned. In the 

 anterior half of the germ-shield, representing ths future 

 head, there is no separation between the inner primitive 

 vertebral mass and the outer side-layers. 



During these processes, this intestinal-glandular layer, 

 the inner germ-layer, remains quite unaltered ; no separations 

 are to be seen in it (Figs. 90, dd, 91, d). The changes, there- 

 fore, which take place at this period in the skin-sensory 

 layer, the outer germ-layer, are all the more remarkable. The 

 continuous elevation and growth of the dorsal swellings tends 

 to make the upper, free margins of these prominent ridges 

 incline towards each other, and as they continually ap- 

 proach each other (Fig. 91, w), they finally coalesce. Thus 

 the open dorsal furrow, the separation at the top of which 

 grows narrower and narrower, is transformed into a closed 

 cylindrical tube (Fig. 92, mr). This tube is of the greatest 

 importance, for it is the first basis of the central nervous 

 system — the brain and spinal cord. This rudiment is called 

 the medullary tube (tubus medullaris). Formerly this fact 

 was regarded with wonder as an inexplicable enigma, but 



