628 ANATOMY OF VERTEBRATES. 



of the respiratory cavity in their primitive and independent 

 blastema : their communicating duct advances with the elonga- 

 tion of the oesophagus, and at the point of its communication 

 therewith the larynx is ultimately developed. The lungs them- 

 selves extend, as simple elongated sacs slightly reticulated on the 

 inner surface, backward into the abdominal cavity. These recep- 

 tacles are no sooner formed than the larva rises to the surface 

 and swallows air, which passes into and expands the prepared 

 cavity. When the pulmonary respiration has regularly begun, the 

 fore-limbs are liberated from the branchial chamber, which now 

 beo-ins rapidly to contract its dimensions, and to be completely 

 partitioned off from the abdominal cavity with which it had pre- 

 viously communicated. 



The changes in the hyo-branchial ajiparatus, accompanying 

 those of the breathing organs, are defined at p. 90, and illustrated 

 in figs. 69-71 and 74. The developement of the vertebrse is 

 attended with the conversion of biconcave into cup-and-ball 

 joints, by ossification of the substance of the cavities, a, fig. 433, 

 and its coalescence either with the fore {Pipa) or back {Rana) 

 part of the centrum, c. The chief facts in the formation of the 

 skull are stated at p. 86, figs. 68-71. 



About the middle period of aquatic life, the true or permanent 

 kidneys begin to be formed from and upon the primordial ones ; 

 and the basis of the ovaria, or testes, may now be discerned. The 

 oviduct is soon distinct from the ureter ; but the testes retain the 

 same excretory duct as the kidnej'S : their vasa deferentia com- 

 municate with retained ca3ca of the primordial kidneys before 

 jienetrating the later glands : the upj^er or anterior ends of the 

 first remain for some time behind the heart. 



In the often-c[uoted experiments of Edwards,' it is not clearlv 

 shown that the Tadpoles of the Frogs were constantly supplied 

 with proper temperature and food, and therefore it is not satisfac- 

 torily proved that the arrest of the metamorphosis was due solely 

 to the absence of light. INIere absence or diminution of this 

 stimulus does not in all cases check the progress of the tadpole to 

 the Frog-state. Ova of a Frog, deposited on ]\Iarch 11, were 

 placed in a vessel covered witli six or eio-lit folds of black adazed 

 calico in a dark pai'tof a room, but in a temperature of from 55° to 

 65° Falirenheit, and supplied with proper food.'^ The larva? were 

 hatched on March 20 ; attained the length of an inch on ]May 1, fig. 

 438 ; had pushed out tlicir hind-logs, fig. 439, on IMay io, and 

 their fore-legs, fig. 440, on J\Iay 16 : the tail began to be absorbed 



