VIII.] THE CRANIUM. 235 



walls. They are placed opposite the intervals between 

 the muscle-plates, and are developed independently of 

 the vertebrae, with the transverse processes of which 

 they subsequently become closely united by fibrous 

 tissue. 



The sternum appears to be formed from the fusion 

 of the ventral extremities of a certain number of the 

 ribs. The extremities of the ribs unite with each other 

 from before backwards, and thus give rise to two car- 

 tilaginous bands. These bands become segmented off 

 from the ribs with which they are at first continuous, 

 and subsequently fuse in the median ventral line to 

 form the unpaired sternum. 



The skull. Two distinct sets of elements enter into 

 the composition of the avian skull. These are (1) the 

 cranium proper, (2) the skeleton of the visceral arches. 



The cranium. As we mentioned in the last chap- 

 ter, the formation of the primitive cranium commenced 

 upon the fourth day. This primitive cranium, in its 

 earliest stage, inasmuch as it is composed of condensed 

 but otherwise only slightly differentiated mesoblast, may 

 be spoken of as the membranous cranium. On the sixth 

 day true hyaline cartilage makes its appearance as a 

 differentiation within the membranous cranium. The 

 cartilaginous cranium is composed of the following parts. 



(1) A pair of cartilaginous plates placed on each 

 side of the cephalic section of the notochord, and known 

 as the parachordals (Fig. 76, iv.). These plates, together 

 with the notochord (nc.) enclosed between them, form a 

 floor for the hind- and mid-brain. The continuous plate, 

 formed by them and the notochord, is known as the 

 basilar plate. 



