272 HAND-BOOK OF PHYSIOLOGY. 



the vertebrae make their appearance, in the latter their arches, which 

 enclose the neural canal. 



The vertebrae do not exactly correspond in their position with the pro- 

 tovertebrae: but each permanent vertebra is developed from the contigu- 

 ous halves of two protovertebrae. The original segmentation of the pro- 

 tovertebrae disappears and a fresh subdivision occurs in such a way that a 

 permanent invertebral disc is developed opposite the centre of each pro- 

 tovertebra. 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 ven- 

 tral portions of the protovertebrae, as we have already seen, give rise to 

 the vertebrae and heads of the ribs, but the outer part of each also gives 

 rise to a spinal ganglion and nerve-root. 



The chorda is now enclosed 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 downward and forward 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 

 outgrowths, one from the foetal brain, which grows downward, and the 

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

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

 nection of the first process with the brain becomes narrowed, and persists 

 as the infundibulum, while that of the other process with the buccal 



