614 THE REPRODUCTIVE FUNCTIONS. 



an extensive ramified system of branchial arte- 

 ries and veins. Still all these are merely tem- 

 porary structures ; for when the period of change 

 approaches, and the branchiae are to be super- 

 seded in their office, every vessel, one after 

 another, becomes obliterated ; and there remain 

 only the two original aortae, which unite into a 

 single trunk lower down, and from which pro- 

 ceed the pulmonary arteries, conveying either the 

 whole, or a portion of the blood, to the newly 

 developed respiratory organs, the lungs. 



By a similar process of continued bifurcation, 

 or the detachment of branches in the form of 

 loops, new vessels are developed in other parts 

 of the body ; as has been particularly observed 

 in the finny tail, and the external gills of the 

 frog, and the newt, parts which easily admit of 

 microscopical examination.* 



Progress is in the mean while making in the 

 building of the skeleton ; the forms of the prin- 

 cipal bones being modelled in a gelatinous sub- 

 stance, which is converted into cartilage ; begin- 

 ning at the surface, and gradually advancing 

 towards the centre of each portion or element of 

 the future bone ; and thus a temporary solid 

 and elastic scaffolding is raised, suited to the 



* Such is the result of the concurring observations of Spallan- 

 zani, Fontana, and Dcillinger. 



