204 THE FROG. 



inwards to form the floor of the skull, while their outer edges 

 grow upwards, forming low ridges along the sides of the brain 

 in the ethmoidal region, in front of the eye, and in the occipital 

 region behind the ear. Opposite the eyes, the side walls of the 

 cranium are formed by a pair of cartilaginous plates, which appear 

 independently, in the membrane at the sides of the brain, and 

 soon extend ventralwards to meet, and fuse with, the trabeculae. 



The side walls and roof of the hinder part of the cranium are 

 mainly formed by the auditory capsules. At an early period 

 the mesoblast surrounding each auditory vehicle forms a definite 

 capsule around it : in this a thin shell of cartilage is formed, 

 first in the outer wall, but soon spreading round to the floor, 

 where it becomes continuous with the outer edge of the para- 

 chordal cartilage (Figs. 70, 90, 91, EC). This cartilaginous 

 auditory capsule spreads more slowly over the dorsal surface of 

 the auditory vesicle, and then extends downwards, between the 

 brain and the ear, to form the side wall of this part of the 

 skull. Later still, in tadpoles of about 20 mm. length, the 

 cartilage of the dorsal borders of the auditory capsules spreads 

 inwards across the top of the brain case, and so completes its 

 roof in this region. 



The hindmost end, or occipital region of the skull, is formed by 

 upgrowth of the edges of the parachordal cartilages. This occurs 

 very slowly, and quite independently of the auditory capsules ; it 

 is not completed until shortly before the metamorphosis. 



In the outer wall of each auditory capsule, a large oval or 

 circular patch remains unchondrified : this is the fenestra ovalis 

 (Fig. 90, SA). In the middle of this membranous patch the stapes 

 appears, in tadpoles of about 16 mm. length, as a cartilaginous 

 nodule (c/. Fig. 93, SA), the further development of which will 

 be considered later. 



At the anterior end of the cranium, the nasal or olfactory 

 organs become roofed in by cartilage, which arises as a vertical 

 crest from the upper surface of the median internasal plate 

 formed by the trabeculas, and spreads out right and left as a 

 pair of thin horizontal plates, covering over the olfactory organs, 

 and forming the cartilaginous olfactory capsules (Fig. 93, OF). 



It thus appears that the trabeculas, with their posterior con- 

 tinuations, the parachordals, give rise to the floor of the cranium 

 along its whole length ; they also, by upgrowth of their edges, 



