REPRODUCTION AND DEVELOPMENT 55 



studied and, in addition, the external gills which are appearing as a 

 tuftlike growth on each gill plate. Draw, orienting as before. 



(i) If living tadpoles are available observe their movement as they 

 lie on their sides on the bottom of the dish. The ectodermal cilia 

 are still present but will gradually disappear. 



Exercise 60.— The Third Tadpole Stage. 



(j) Up to this time the tadpole, although hatched, swims but little. 

 Instead, it clings to the jelly and other objects by means of its sucker. 

 Has the structure of the sucker changed? Now the activity becomes 

 greater and in the living specimens swimming is much more in evi- 

 dence. Study specimens with well-developed external gills, identify- 

 ing all the parts as above and, in addition, the stomodeum which now 

 appears as a median pit between the sucker and the eyes. It is seen 

 clearly from the ventral view. To what does it give rise? Where is 

 the proctodeum at this stage? The brain vesicles can be seen through 

 the superficial ectoderm as swellings above the line between eye and 

 gills. Look for mesodermal somites. What is the distribution of the 

 pigment at this stage? Draw, orienting as before. 



Exercise 61. — The Fourth Tadpole Stage. 



(k) In older tadpoles look for the operculum, a membrane com- 

 parable to the lateral covering of the gills of a fish. It will be found 

 overgrowing the external gills. The external gills are temporary respir- 

 atory organs and soon disappear with the growth of the internal gills 

 which are in the region covered by the operculum and are comparable 

 to the gills of a fish. Draw a tadpole, showing the external gills 

 partially overgrown by the operculum. 



G. The Late Larval Stages 



Exercise 62. — External Features. 



(a) Living specimens should be examined in an aquarium. Notice 

 how they come up to breathe a,nd how those in the more advanced 

 stages are beginning to use their legs. For detailed study, preserved 

 tadpoles of the leopard frog or of the bullfrog may be used. Select 

 a specimen with hind legs just appearing. It should be placed under 

 water in a dissecting pan. Identify the nostrils, the eyes, the mouth 

 with its horny jaws which function as teeth, the hind legs, and, on the 

 left side of the body, the opercular opening which leads into the gill 

 chamber. Draw from a side view, orienting as in previous figures, 

 and showing the opercular opening by a dotted outline if your drawing 

 is from the right side. 



