432 THE BABBIT. 



from the proximal end of the rib, and does not articulate with 

 the vertebra until a considerably later period. The posterior ribs, 

 behind the first two or three, develop from the start indepen- 

 dently of the vertebrae. 



The sternum is formed in two halves, and from the ventral 

 ends of the ribs. Each rib is at first slightly dilated at its 

 ventral end (Fig. 163, ST), and these enlarged ends of successive 

 ribs, growing both anteriorly and posteriorly, meet and fuse, so 

 as to form along either side a longitudinal cartilaginous bar, 

 connecting the ventral ends of the ribs of its side of the body. 

 The two bars, right and left, approach each other, meet in the 

 median plane, and fuse to form the sternum. 



3. The Skull (cf. Figs. 160 and 162). 



The cartilaginous skull of the rabbit is formed from the same 

 essential elements parachordals, trabeculas, sense capsules, and 

 visceral bars as in the chick or frog ; and the general relations 

 of these parts to one another are very similar in the three animal?. 

 Cartilage appears in the head of the rabbit embryo about the 

 fourteenth day, and by the sixteenth or seventeenth day the 

 cartilaginous skull is practically completed. 



The two parachordal cartilages fuse together very early to 

 form the basilar plate (Fig. 151, RP), which underlies the medulla 

 oblongata, and forms the floor of the hinder part of the skull. 

 The edges of the basilar plate grow up at the sides of the brain, 

 and fuse with the independently arising periotic capsules 

 (Fig. 159, EC) ; and then, growing in towards each other, meet 

 above the cerebellum to complete the occipital ring (Fig. 151, 

 ox). In front of the supra-occipital cartilage, the roof of the 

 skull remains membranous until the formation of the bones. 



The trabeculae are a pair of rods of cartilage, which are con- 

 tinuous at their hinder ends with the basilar plate : further 

 forwards they lie at the sides of the pituitary body, and in 

 front of this unite to form the ethmoidal plate (Fig. 151, ET). 

 This latter is at first small, and never becomes so large as in the 

 bird, but as the nose grows forwards, and the face assumes its 

 definite form, the ethmoidal plate extends forwards with it, giving 

 off from its upper surface a median vertical septum between the 

 two olfactory organs. The cartilaginous olfactory capsules arise 

 independently, but very early fuse with the ethmoidal septum. 



