680 ' HANDBOOK OF PHYSIOLOGY. 



and also upwards between the medullary canal and the epiblast covering 

 it. In the former situation, the cartilaginous bodies of the vertebrae 

 make their appearance, in the latter their arches, which inclose the neu- 

 ral canal. 



The vertebrae do not exactly correspond in their position with the 

 protovertebrae: but each permanent vertebra is developed from the con- 

 tiguous halves of two protovertebrae. The original segmentation of the 

 protovertebrae disappears, and a fresh subdivision occurs in such a way 

 that a permanent invertebral disc is developed opposite the centre of 

 each protovertebra. Meanwhile the protovertebrae split into a dorsal 

 and ventral portion. The former is termed the musculo-cutaneous plate, 

 and from it are developed all the muscles of the back together with the 

 cutis of the dorsal region (the epidermis being derived from the epiblast). 

 The ventral portions of the protovertebrae, as we have already seen, give 

 rise to the vertebrae and heads of the ribs. 



The chorda is now inclosed in a case, formed by the bodies of the 

 vertebrae, but it gradually wastes and disappears. Before the disappear- 

 ance of the chorda, the ossification of the bodies and arches of the verte- 

 brae begins at distinct points. 



The ossification of the body of a vertebra is first observed at the point 

 where the two primitive elements of the vertebrae have united inferiorly. 

 Those vertebrae which do not bear ribs, such as the cervical vertebrae, 

 have generally an additional centre of ossification in the transverse pro- 

 cess, which is to be regarded as an abortive rudiment of a rib. In the 

 foetal bird, these additional ossified portions exist in all the cervical ver- 

 tebrae, and gradually become so much developed in the lower part of the 

 cervical region as to form the upper false ribs of this class of animals. 

 The same parts exist in mammalia and man; those of the last cervical 

 vertebrae are the most developed, and in children may, for a considerable 

 period, be distinguished as a separate part on each side like the root or 

 head of a rib. 



The true cranium is a prolongation of the vertebral column, and is 

 developed at a much earlier period than the facial bones. Originally it 

 is formed of but one mass, a cerebral capsule, the chorda, dorsalis being 

 continued into its base, and ending there with a tapering point. At an 

 early period the head is bent downwards and forwards round the end of 

 the chorda dorsalis in such a way that the middle cerebral vesicle, and 

 not the anterior, comes to occupy the highest position in the head. 



Pituitary Body. In connection with this must be mentioned the 

 development of the pituitary body. It is formed by the meeting of two 

 out-growths, one from the foetal brain, which grows downwards, and the 

 other from the epiblast of the buccal cavity, which grows up towards it. 

 The surrounding mesoblast also takes part in its formation. The con- 

 nection of the first process with the brain becomes narrowed, and per- 



