GENERATION AND DEVELOPMENT. 273 



cavity disappears completely at a spot corresponding with the future posi- 

 tion of the body of the sphenoid. 



The first appearance of a solid support at the hase of the cranium ob- 

 served by Miiller 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 enclose 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 developed at 

 the sides of the chorda dorsalis two symmetrical elements, which subse- 

 quently coalesce, and may wholly enclose the chorda. 



The brain-case consists of three segments: occipital, parietal, and 

 frontal, corresponding in their relative position to the three primitive cer- 

 ebral vesicles; it may also be noted that in front of each segment is devel- 

 oped a sense-organ (auditory, ocular, and olfactory, from behind forward). 

 The basis cranii consists at an early period of an unsegmented cartilagi- 

 nous 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 pre-sphenoid, one corresponding to each 

 segment. 



The bones forming the vault of the skull (frontal, parietal, squamous 

 portion of temporal), with the exception of the squamo-occipital, which 

 is pre-formed in cartilage, are ossified in membrane. 



DEVELOPMENT OF THE FACE AND VISCEKAL ARCHES. 



It has been said before that at an early period of development of the 

 embryo, there grow up on the sides of the primitive groove the so-called 

 dorsal laminw, which at length coalesce, and complete by their union the 

 spinal canal. The same process essentially takes place in the head, so 

 as to enclose the cranial cavity. 



Visceral Laminae. The so-called visceral laminae have been also 

 described as passing forward, and gradually coalescing in front, as the 

 dorsal laminae do behind, and thus enclosing the thoracic and abdominal 

 cavity. An analogous process occurs in the facial and cervical regions, 

 but the enclosing laminae, instead of being simple, as in the former 

 instances, are cleft. 



In this way the so-called visceral arches and clefts are formed, four on 

 each side (Fig. 430, A). 



From or in connection with these arches the following parts are de- 

 veloped; 



VOL. II. 18. 



