614 THE REPRODUCTIVE FUNCTIONS. 



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 Dollinger. 



