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from that layer invade the notochord sheath ; it becomes 

 cartilaginous, and (excepting the anterior portion) it is 

 divided into segments which thicken centrally (centra of 

 the vertebrae), and the notochord is thus constricted and 

 reduced. The neural and haemal arch rudiments arise 

 as outgrowths from the " skeletogenous layer " ; and 

 from their bases the tissue spreads over the centra, which 

 are therefore formed by segmentation of the chondrified 

 notochord sheath followed by overgrowth of skeletogenous 

 (arch-rudiment) tissue. 



In the tail region the ventral arch rudiments form a 

 haemal arch ; but in the trunk region, they form pro- 

 jecting transverse processes from which a portion (the 

 rib) becomes jointed off. 



Give a General Description of the Skull and its Development. 



The skull or cranium supports the parts of the head, 

 and forms a protective case for the brain and sense organs. 

 Its primitive form is that of a cartilaginous trough, the 

 chondrocranium, and the mode of development (from 

 mesenchyme) is mainly as follows : 



Beneath the brain, cartilages are developed, namely, 

 two, the trabeculae, which become or are already continuous 

 with other two, the parachordals. The parachordais ariso 

 one along each side of the front part of the notochord, 

 and in part represent the unsegmented portion of its sheath. 

 By growth and fusion these cartilages form a floor, which 

 has a hollow for the infundibulum of the brain ; the part 

 in front of the hollow (formed by the trabeculae) is the 

 ethmoid region, and the part behind (formed by the para- 

 chordals) is the basilar plate. 



Continued chondrification, from the floor upward, forms 

 the side walls and roof (incomplete). The cartilaginous 

 capsules around the nose and ear, which have a more or 

 less independent origin, become completely united with 

 the chondrocranium ; the nasal capsules are fused with the 

 ethmoid region, and the auditory capsules are fused with 

 the occipital (notochordal) portion of the basilar plate to 

 form the back part of the brain-case. 



The principal regions, now established, are : 



1. The wide anterior or nasal region, with the meseth- 



moid between the olfactory capsules. 



2. The narrow interorbital or sphenoidal region between 



the orbits (i.e., incurvinsrs of the side walls to 

 form sockets for the eyes). 



