THE PEOTOVEETEBRiE. 11 



surface of the anterioi* vesicle lies in a kind of pit, in front 

 of, and rather below, the apex of the notochord, and the 

 pituitary gland is developed in connection with it. From 

 the opposite upper surface of the same vesicle the pineal 

 gland is evolved, and the part of the anterior cerebral vesicle 

 in connection with which these remarkable bodies arise, 

 is the future third ventricle. 



Behind, the posterior cerebral vesicle passes into the 

 primitively tubular spinal cord (Fig. 4, A). Where it does 

 so, the head ends, and the spinal column begins ; but no 

 line of demarcation is, at first, visible between these two, 

 the indifferent tissues which ensheath the notochord pass- 

 ing, without inteiTuption, from one region to the other, and 

 retaining the same character throughout. 



The first essential differentiation between the skull and 

 the vertebral column is effected by the appearance of the 

 protovei-tehro'.. At regular intervals, commencing at the 

 anterior part of the cervical region, and gradually extending 

 backwards, the indifferent tissue on each side of the 

 notochord undergoes a histological change, and gives rise 

 to more opaque quadrate masses, on opposite sides of the 

 notochord (Fig. 2, B, 0). Each pair of these gradiially 

 Ignite above and below that structure, and send arched pro- 

 longations into the walls of the spinal canal, so as to con- 

 stitute a protovertebra. 



No protovertebrse appear in the floor of the skiill, so 

 that, even in this early stage, a clear distinction is drawn 

 between the skull and the spinal column. 



The Spinal Sijstem. — The protovertebrse consist at first 

 of mere indifferent tissue ; and it is by a process of histo- 

 logical differentiation within the protoverfebral masses, 

 that, from its deeper parts, one of the spinal ganglia and 

 a cartilaginous vertebral centrum. — from its superficial layer, 

 a .segment of the dorsal muscles, are produced. 



Chondrification extends upwards into the walls of the 

 dorsal tube, to prodiice the ne^lral arch and spine of each 

 vertebra ; and, outwards, into the wall of the thoracic and 



