DEVELOPMENT OF THE COMMON FROG. 117 



these are the sucker and the external gills ; and they are 

 already so far developed as to be perfectly efficient when 

 the tadpole escapes from the egg-membrane. The sucker 

 is at first large and single, extending across the middle 

 line ; and it is in this condition when the animal is hatched. 

 The external gills, two on each side, have budded out from 

 the skin over the middle of the first and second branchial 

 arches, and have, before hatching, attained such a size as 

 to serve at once as organs of respiration. There is no 

 regularity in their form or in the number and arrange- 

 ment of their divisions ; most frequently they consist of 

 an irregular bunch of filaments. The time occupied by 

 this period was, in the present case, fifteen days. 



Second Period. — The special temporary organs in use 

 during this period are the sucker and the external gills. 

 The developments which take place in it are those of the 

 branchial arches and clefts, the internal gills, the eyes, 

 the gill-chambers, formed by the opercular fold, which 

 commences to grow on the second day and is completed 

 on the fourteenth ; the passages of the nostrils to the back 

 of the mouth, these being in connexion with the internal 

 gills, the ciliary action of which draws the water through 

 them ; and the suctorial beaked mouth and coiled intes- 

 tine peculiar to the tadpole. At the close of the period 

 very small rudiments of hind limbs appear. 



This second period is a continuation of the embryonic 

 condition ; contact with water and aquatic respiration are 

 necessary for further development; but rest is also re- 

 quired, and the animal continues as stationary as whilst 

 in the egg. It is blind and helpless and cannot feed, 

 having no alimentary canal. Immediately on its escape 

 the tail is used, but only to swim to some fixed object, to 

 which it attaches itself by its sucker ; and there it remains 

 until the close of the period, unless forcibly detached, when 

 it at once refixes itself. 



