DEVELOPMENT. 681 



sists as the infundibulum, while that of the other process with the buc- 

 cal 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 ob- 

 served by Muller in fish, consists of two elongated bands of cartilage 

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

 are connected with the cartilaginous capsule of the auditory apparatus, 

 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 de- 

 veloped at the sides of the chorda dorsalis two symmetrical elements, 

 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 

 forwards). The basis crauii consists at an early period of an unseg- 

 mented cartilaginous rod, developed round the notochord, and continued 

 forward beyond its termination into the trabeculce cranii, which bound 

 the pituitary fossa on either side. 



In this cartilaginous rod three centres of ossification appear: basi- 

 occipital, basi-sphenoid, and p re-sphenoid, one corresponding to each 

 segment. 



The bones forming the vault of the skull, viz., the frontal, parietal, 

 squamous portion of temporal and the squamo-occipital, are ossified in 

 membrane. 



The Visceral Clefts and Arches. 



As the embryo enlarges, the heart, which at first occupied a position 

 close to the cranial flexure, is carried further and further backwards 

 until a considerable intervening part exists between it and the head, in 

 which the mesoblast is undivided. This becomes the neck. On a sec- 

 tion it is seen that in it the whole three layers are represented in order, 

 and that there is no interval between them. In the neck thus formed 

 soon appear the visceral or branchial clefts on either side, in series, 

 across the axis of the gut not quite at right angles. They are four in 

 number, the most anterior being first found. At their edges the hypo- 

 blast and the epiblast are continuous. The anterior border of each cleft 

 forms a fold or lip, the branchial or visceral fold. The posterior bor- 

 der of the last cleft is also formed into a fold, so that there are four 

 clefts and five folds, but the three most anterior are far more prominent 



