230 THE DEVELOPMENT OF THE SKULL. [CHAP. 



and consists of a rod on eacli side. Each of these early 

 becomes divided into two parts, a proximal and a distal. 

 From the bones which subsequently develope in them the 

 proximal one is known as the pterygoid (Fig. 69, 'pg), and 

 the distal one as the palatine (Fig. 69, pa). Both of the 

 rods are very delicate, and remain in the cartilaginous con- 

 dition for a short time only. 



8. In the mandibular arch itself, there is also a proxi- 

 mal and distal cartilage. The proximal cartilage is situated 

 (Figs. 68 and 69, q) at the side of the investing mass but 

 not united with it. It is known as the quadrate, and in the 

 early stage is merely a small knob of cartilage. The distal 

 rod is called Meckel's cartilage (Fig. 69, mk) ; on it are sub- 

 sequently moulded the bones which form the mandible; 

 while its proximal end becomes the articulare. 



9. In the next arch, usually called the second visceral 

 or hyoid arch, there is a very small development of cartilage. 

 This consists of a central azygos piece the 'hasi-hyaV (Fig. 69, 

 hh), and two rods, one on each side, the ' cerato-hyals' (Fig. 69, 

 ch). 



10. In the third arch, which corresponds with the first 

 branchial arch of the Ichthyopsida, there is on each side a 

 large distal cartilaginous rod (Fig. 69, cbr) the ' cerato- 

 branchial,' and a smaller proximal piece (Fig. 69, ehr) ; be- 

 tween the two arches lies an undefined mass (Fig. 69, bbr) 

 the ' basibranchtal.' In the arches behind this one there is 

 in the bird no development of cartilage, 



11. The growth of this primordial cartilaginous skull is 

 very rapid, and by the 5th or 6th day very important changes 

 have taken place. In the first place the pre-pituitary portion 

 of the cranio-facial axis becomes equal to and subsequently 

 actually surpasses in length the post-pituitary part. This 

 change is accompanied by a considerable decrease in the 

 cranial flexure. In the investing mass, the chief changes 

 are an upward growth on each side behind, to form the exoc- 

 cipitals ; and the appearance of the occipital condyles (Fig. 

 70, oc) as small swellings on each side of the middle line 

 at the hind end of the plate. In front of the termination 

 of the notochord two transverse vertical walls of cartilage 

 rise up, the one in front of and the other behind the pitui- 

 tary body: these are called respectively the anterior and 



