614 



THE EMBRYOLOGY OF THE FROG 



and extending anteriorly as parachordae, or parachordal cartilages. 

 These parachordal cartilages now fuse with the posterior ends of the 

 trabeculae to enclose the tip of the notochord, and the entire continuous 

 plate beneath the fore-brain is then called the parachordal plate. 



These parts can be made understandable only by a careful examina- 

 tion of Figures 310 and 353, which must be studied with great thorough- 

 ness or much of our later work in comparative anatomy will be valueless. 



From the visceral arches, the palato-quadrates are formed as a pair 

 of flattened rods, lateral to the trabeculae. These are in intimate rela- 



Fig. 353. 



A. Chondrocranium of 29 mm. larva of R. fusva. To the left, the ventral 

 surface ; to the right, the dorsal surface, a, Auditory capsule ; bp, basal plate ; c, 

 notochord ; ct, trabecular cornu ; /, basicranial fontanelle ; fa, foramen for carotid 

 artery ; fin, foramen magnum ; fo, foramen for olfactory nerve ; ir infrarostral 

 cartilage ; j, jugular foramen for IX and X cranial nerves ; I, perilymphatic 

 foramina ; m, muscular process ; M, Meckel's cartilage ; o, otic process of palato- 

 quadrate ; pf, palatine foramen ; pg, palato-quadrate cartilage ; sr, suprarostral 

 cartilage ; t, trabecular cartilage ; v, secondary fenestra vestibuli. B, Anterior 

 portion of chondrocranium of JR. fusca during metamorphosis. Lateral view. 

 C. Skull of 2 cm. R. fusca, after metamorphosis. Dorsal view. Membrane bones 

 removed from left side, a, Auditory capsule ; am, anterior maxillary process ; 

 on, annulus tympanicus ; art, articular process of palato-quadrate cartilage ; eo, 

 exoccipital bone ; /, fronto-parietal bone ; fpo, prootic foramen ; mx, maxillary 

 bone ; n, nasal bone ; o, olfactory cartilages ; on, orbito-nasal foramen ; pa, 

 anterior ascending process of palato-quadrate ; pg, pterygoid bone ; pi, plectrum ; 

 pm, posterior maxillary process ; pp, posterior ascending process of palato- 

 quadrate ; pq, palato-quadrate cartilage ; pt, pterygoid process of palato-quadrate ; 

 px, premaxillary bone ; qj, quadrato-jugal bone ; II, foramen for optic nerve ; 

 ///, foramen for III cranial nerve; IV, foramen for IV cranial nerve. (From 

 Ziegler.) 



tion to the cranium proper. They connect with the trabeculae by an- 

 terior ascending processes back of the olfactory region, and by posterior 

 ascending processes opposite the end of the notochord. 



The remaining portion of the skull which develops from the visceral 

 arches is connected with the jaw, and will be described shortly. 



The infundibulum and pituitary body lie within the basi-cranial 

 fontanelle, which is the open space just anterior to the tip of the noto- 

 chord. From now on, development continues mostly in the posterior 

 portion of the cranium. 



Figures 310 and 353 show how the auditory organ is formed by a 

 connective tissue capsule which soon becomes cartilage, while the 

 mesotic cartilage grows out posteriorly and laterally from the para- 



