328 THE CHICK. 



In a seven-day chick embryo there are forty-five vertebra? 

 present, of which the hindmost five or six fuse at a later stage to 

 form the pygostyle. 



Of the ' sacral ' vertebras, the first four have ribs, which in the 

 first are long, and in the remaining three are much shorter. The 

 fifth, sixth, seventh, and eighth vertebras have no ribs ; the ninth 

 and tenth have ribs, and are for this reason regarded by many 

 zoologists as the true sacral vertebras. The remaining five, or 

 ' urosacral,' vertebras have no rib elements. Up to the seventh 

 day these latter are quite distinct from the ilia, which stop at the 

 tenth vertebra of the sacral series : in the later stages the ilia 

 gradually extend further backwards, and ultimately overlap and 

 fuse with all five ' urosacral ' vertebras. 



2. The Skull. 



The skull of the chick consists of the same morphological 

 elements as that of the frog, viz. : 



(i) The cranium, or brain case. 



(ii) The sense capsules, olfactory and auditory. 



(iii) The visceral skeleton. 



The sense capsules and the cranium are, however, so closely 

 united from their earliest appearance that it will be convenient 

 to describe them together. 



The main factors of the skull may be recognised, in the form 

 of tracts of condensed mesoblast, as early as the fourth day, but 

 it is not until the sixth day that cartilage is definitely established. 



a. The Cartilaginous Skull. 



(i) and (ii) The Cranium and the Sense Capsules. 



At the end of the sixth day, when cartilage first appears in 

 definite form, the structure of the skull is as follows (cf. Figs. 

 116 and 123). The notochord extends forwards in the median 

 plane, beneath the brain, as far as the hinder end of the pituitary 

 body, where it stops. At the sides of the notochord, and in close 

 contact with it, are a pair of horizontal cartilaginous plates, the 

 parachordal plates, which, with the notochord, form a broad floor 

 to the hinder part of the skull, underlying the hind- and mid- 

 brains. Imbedded in the parachordal plates, and continuous 

 with them from their first appearance, are the cartilaginous 

 auditory capsules, inclosing the auditory organs. 



