430 THE KEPRODUCTIVE FUNCTIONS. 



By a similar process of continued bifurcation, or the de- 

 tachment of branches in the form of loops, new vessels are 

 developed in other parts of the body, as has been particu- 

 larly observed in the finny tail, and the external gills of the 

 frog, and the newt, parts which easily admit of microscopi- 

 cal 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 scaffolding is raised, suit- 

 ed to the yielding texture of the nascent organs: lastly, the 

 whole fabric is surrounded 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 continuous integument. The eyes, 

 which were hitherto unprotected, receive special means of 

 defence, by the addition of eyelids, which are formed by a 

 farther 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 this hard shell, which had hitherto afforded it 

 protection, is now opposed to its emancipation; and the 

 chick, in order to obtain its freedom, must, by main force, 

 break through the walls of its prison; its beak is, however, 

 as yet too tender to apply the force requisite for that pur- 

 pose. Here, again, we find Nature expressly interposing 

 her assistance; for she has caused a pointed horny projection 

 to grow at the end of the beak, for the special object of 

 giving the chick the power of battering its shell, and making 

 a practicable breach, through which it shall be able to creep 

 out, and begin its new career of life. That this horn is pro- 



• Sucli is the result of the concurring observations of Spallanzani, Fonta- 

 na, and DoUinger. 



