DEVELOPMENT. 791 



inferiorly. Those vertebrae which do not bear ribs, such as the cer- 

 vical vertebras, have generally an additional centre of ossification in 

 the transverse process, which is to be regarded as an abortive rudi- 

 ment of a rib. In the foetal bird, these additional ossified portions 

 exist in all the cervical vertebrae, 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 chil- 

 dren 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 per- 

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

 cavity disappears completely at a spot corresponding with the future 

 position of the body of the sphenoid. 



Cranium. The first appearance of a solid support at the base of the 

 cranium observed by Muller in fish, consists of two elongated bands of car- 

 tilage (trabeculae cranii), one on the right and the other on the left side, 

 which are connected with the cartilaginous capsule of the auditory ap- 

 paratus, and which diverge to inclose the pituitary body uniting in 

 front to form the septum nasi beneath the anterior end of the cerebral 

 capsule. Hence, in the cranium, as in the spinal column, there are at 

 first developed at the sides of the chorda dorsalis two symmetrical ele- 

 ments, which subsequently coalesce, and may wholly inclose the chorda. 



The brain-case consists of three segments: occipital, parietal, and 

 frontal, corresponding in their relative position to the three primitive 

 cerebral vesicles ; it may also be noted that in front of each segment is 

 developed a sense-organ (auditory, ocular, and olfactory, from behind 

 forward). The basis cranii consists at an early period of an unsegmented 

 cartilaginous rod, developed round the notochord, and continued for- 

 ward beyond its termination into the trabeculce cranii^ which bound the 

 pituitary fossa on either side. 



