ORGANIC DEVELOPEMENT. 551 



proceed 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 principal 

 bones being modelled in a gelatinous substance, 

 which is converted into cartilage; beginning 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 scaf- 

 folding is raised, suited to the yielding texture of 

 the nascent organs : lastly, the whole fabric is sur- 

 rounded by an outer wall, the building of which 

 is begun from the dorsal region, and conducted 

 round the sides of the body, till the two portions 

 come to meet in the middle abdominal line, where 

 they are finally united into one general and con- 

 tinuous integument. The eyes, which w ere hitherto 

 unprotected, receive special means of defence, by 

 the addition of eyelids, which are formed by a 

 further extension and folding of these integuments ; 

 and the greater part of the surface of the body 

 gives rise to a growth of temporary down, which, 

 as we have seen, is provided as a covering to the 

 bird at the time it is ready to quit the shell. But 



* Such is the result of the concurring observations of Spalianzani, 

 Fontana, and Dollinger. 



