THE DEVELOPMENT OF THE FOWL'S SKULL. 141 



on each side, running from the root of the first visceral arch 

 beneath the eye to the nasal sac, a ridge or elevation, which is 

 called the maxillary process, and might be regarded as a visceral 

 arch of the anterior division of the skull, from the base of which 

 it is developed (Fig. 32, G, and I, Fig. 57, F). 



Lastly, the middle part of the floor of the anterior cerebral 

 vesicle, between the nasal sacs, thickens and gives rise to a 

 broad, flat median process, with an expanded extremity, the 

 terminal contour of which is excavated and slightly produced 

 at the angles — the fronto-nasal process (Fig. 57, F, h). 



At first, the cranium and all its arches are membranous, or 

 composed of mere indifferent tissue, with the exception of the 

 axial notochord; but, very early, chondrification commences. 

 The indifferent tissue surrounding the notochord (the " invest- 

 ing mass" of Kathke) (Fig. 57, 0, D,/), is converted into carti- 

 lage, and the same histological change takes place in the Avails 

 of the auditory capsules, and around the foramen magnum ; the 

 cartilage stops in the middle line, behind the pituitary body, 

 but sends two processes, one on each side of that body, into the 

 floor of the anterior division of the skull (Fig. 57, F 1 , tr). These 

 processes, the trabecule cranii, of Kathke, unite in front, and 

 the cartilage formed by their union ends in the fronto-nasal 

 process. The roof of the skull, and the greater part of its side- 

 walls, except in the region of the foramen magnum, are, at first, 

 entirely membranous. Chondrification next takes place in the 

 visceral arches ; a rod of that substance, which coalesces with 

 its fellow in the middle line, being formed in the axis of the 

 several arches on each side. 



Purposing to return to the visceral arches by and by, I 

 shall now trace out the modifications which are undergone by 

 the chondro-membranous brain-case. In the occipital region, 

 and about the auditory capsules, which early attain a very large 

 proportional size, the cartilage extends for some distance upon 

 the infero-lateral parietes of the skull ; on the floor of the 

 posterior division of the skull it thickens notably, and forms a 

 sort of model of the future basi-occipital and basi-sphenoidal 

 regions, the interspace between the trabecule becoming rapidly 

 obliterated and converted into the floor of the pituitary fossa. 



