42 ANATOMY OF VERTEBRATES. 



greater size and bending down. The notocliord extends into the 

 hind part of the future basis cranii, its gelatinous axis terminating 

 at the bend ; but the blastemal capsule — the true seat of the 

 histoloo^ical chano^es resultina- in vertebral structure — is continued 

 forward, expanding, di^dding below the part of the vertical cere- 

 bral canal called ' infundibulum,' and again uniting anteriorly to 

 form a vertical plate extending between the eye-capsules and be- 

 coming lost in the deflected fore part of the cephalic blastema. At 

 this stage neural segments are not shown. The haemal ones appear 

 as the so-called ' visceral arches ' of the head. The foremost is in- 

 complete below at the M^lastemic' stage, and is represented by a 

 pair of obtuse lobes or buds beneath the eyes ; the next is larger 

 and becomes closed below : a third, a fourth, and a feeble indi- 

 cation of a fifth, correspond mth the primitive vascular arches, 

 and are more truly ' visceral ' than ' vertebral.' Of the latter 

 sio:nificance is that which descends on each side the heart itself, 

 and is soon indicated by the buds of the appendages which become 

 articulated with such * scapular' arch. 



The cartilage formed round the fore part of the notocliord, 

 extends neurad, and attains great thickness at the sides of the 

 cranium in connection with that of the acoustic capsules ; it 

 becomes thinner as it rises, and the primitive tissue closes the ex- 

 panded cranial cavity. The cartilage behind the ear-capsule is of 

 the liindmost neurapophysis : that in front of the capsule is of the 

 next ; that which is formed at the optic foramen is the third in 

 advance : these latter neurapophysial cartilages are formed in the 

 blastemal walls of the cranium distinct from the notochordal car- 

 tilage. This, advancing along the base of the skull, follows the 

 disposition of the extensions of the notochordal capsule, and 

 bifurcates into the so-called 'trabecul^e' (vol. i. figs. 58-60, 5, 5), 

 which again unite to form the basis of the neural arch and apex 

 of the haemal arch of the foremost segment of the skull ; it becomes 

 compressed betAveen the eyes, and expands in advance of them, 

 the end of the ha3mal closing up to that of the neural arch in a 

 Avay which reminds one of the modification of the vertebral axis at 

 the opposite end of the column. Here, however, the nature of 

 the Bird overrides that of the Vertebrate, and every subsequent 

 step in cranial developement relates to adaptive conditions of ver- 

 tebral elements and appendages. Distinct cartilages in the buds 

 or piers of the foremost haemal arch form the basis of the palato- 

 maxillary bones. The palatine cartilage arches outward and 

 backward, like that marked 24 in fig. 60, voL i., and in it is 

 developed the pterygoid. The cartilage in the second arch forms 



