I.] THE COMMON FROG. 



Concomitantly with the development of this canal, 

 there is found, immediately beneath it, a little gela- 

 tinous rod enclosed in a membraneous envelope, and 

 called the notocJiord, or chorda dorsalis. It is this 

 structure which is subsequently developed and be- 

 comes the backbone. 



Another singular condition is invariably presented 

 in the development of every vertebrate, whether the 

 structures formed are transitory or permanent. 



This condition is the appearance of a certain series 

 of openings formed at the side of the neck, and which 

 in fishes remain permanent as the gill openings. 

 These openings are termed visceral clefts, and lead 

 from the exterior into the throat. The solid pillars 

 (or intervals) between the clefts arc called visceral 

 arches, and in creatures {e.g. fishes) which develop gills 

 upon them, brancJiial arcJies. 



In all vertebrates again (unlike insects or spiders) 

 there are never more than four limbs, and these are 

 supported by bones, or cartilages, which are clothed 

 externally with flesh, and are not moved by muscles 

 placed ivitJiin the hard parts, as is the case with 

 lobsters, insects, and all their allies. 



The heart in all vertebrates consists of at least two 

 distinct cavities, and sends forth blood into a system 

 of arteries, whence it is brought back again to the 

 heart by other vessels termed the veins. On its way 

 back to the heart, however, some of the veins carry 

 blood to be redistributed in the liver, forming what 

 is called the /^r/<^/ circulation. 



In all the points above enumerated, the Frog (as 

 we shall shortly see) fully agrees with beasts, birds, 



