2 70 COMPARATIVE ANATOMY 



skull are formed during the metamorphosis, except the 

 sphenethraoid, which does not appear till the frog is nearly 

 full grown. The parasphenoid is the first bone to be formed, 

 and it is worth noting that the frontals and parietals, which 

 are separate bones in many Amphibia, are formed as separate 

 bones in the frog, and afterwards fuse together. The jaws are 

 formed from the mandibular bar of the visceral series which, 

 at first vertical, comes to He beneath and parallel to the 

 trabeculse. It unites with the trabeculse in front of and 

 behind the eyeball, forming the so-called subocular bar, and 

 its anterior end is segmented off as Meckel's cartilage, which is 

 the basis of the lower jaw, while the subocular bar forms the 

 basis of the upper jaw, the quadrate cartilage of the adult. 

 During the metamorphosis these parts undergo rather complex 

 changes of position which will not be detailed here. The 

 upper jaw is completed by the formation of the premaxillas 

 and maxillae in the membrane lying outside the subocular bar, 

 and the palatine and pterygoid are formed, also in membrane, 

 in close connection with the anterior attachment of the bar to 

 the trabecula. 



The hyoid and branchial bars undergo complex changes in 

 connection with their primary functions as supports to the 

 gill-arches. Eventually they degenerate and form the hyoid 

 apparatus of the adult, the anterior end of the hyoid bar 

 becoming attached to the skull and forming the so-called 

 anterior cornu of the hyoid. 



The history of the vascular and excretory systems, though 

 important and capable of being treated at great length, can 

 only be briefly described in this place. Their development is 

 very clearly explained in Marshall's book on the frog. 



The heart appears early in development, after the neural folds 

 are closed, but some time before hatching. It is at first a 

 straight tube, but soon becomes twisted to form a single turn 

 of a spiral, and divided by constrictions into chambers which 

 form the auricles and ventricle. The auricle is at first single, 

 and later is divided into two by a septum which grows down 

 from its dorsal wall. The formation of the heart is closely 

 connected with the formation of the pericardial cavity. 



In the region of the pharynx the lateral sheets of mesoblast, 

 prior to the time we are considering, have not extended to the 

 mid-ventral line and have not spHt into somatopleur and 



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