54 THE FROG 



within its envelopes by means of the cilia which are now present on 

 its ectoderm cells should be observed. 



(e) The embryo of the frog is said to hatch when it frees itself 

 from its jelly envelopes. Hatching time is not exactly correlated with 

 the stage of development of the embryo, but varies with the condition 

 of the surrounding envelopes. This is apparently determined by the 

 quality of the water — pond, distilled, or deep well — in which the eggs 

 have been developing. Consequently, the first tadpole stage is some- 

 what indefinite. 



Exercise 58. — The First Tadpole or Hatching Stage. 



(f) When the frog embryo has hatched it is known as a tadpole; 

 it is a larva, that is, it is structurally different from the adult but 

 self-sustaining. Examine specimens in this stage. The head, trunk, 

 and tail are becoming conspicuous. The available specimens will 

 probably be in slightly different stages and if a half-dozen are taken 

 they can be oriented alike and arranged in order to show the develop- 

 mental changes. Study such a series and follow the further differentia- 

 tion of the parts shown in the last figure. The primordia of the eyes 

 will now be seen. Draw a tadpole, which shows the head, trunk, and 

 tail, the gill plates, the eyes, the sucker, and the proctodeum. 



(g) Cut cross sections through the trunk region of an early tadpole, 

 placing the specimen on wet filter paper and using a sharp scalpel or 

 razor blade as with the blastulse and gastrulse. Examine under water, 

 using the low-power objective and reflected light. Good sections will 

 show the dorsal neural tube, the notochord, the archenteron lined 

 with endoderm, the yolk-laden cells, the dorsal mesoderm on each side 

 of the neural tube, the intermediate and lateral mesoderm, which will 

 probably not be distinguished unless the ccelom is visible as the cavity 

 of the lateral mesoderm, and the superficial ectoderm. Depending on 

 the position of your cut, other structures, such as the liver and the 

 pronephroi, or head kidneys, may be seen. Identify as many parts as 

 possible and draw the cut surface. Examine a demonstration section 

 of this stage. 



Exercise 59. — The Second Tadpole Stage. 



(h) Select a number of specimens in which the tail is beginning to 

 differentiate into a fin and a muscular axis. Arrange a series as before. 

 At one stage the tadpole will show metameric markings on each side 

 in the dorsal region. These are the mesodermal somites which are 

 formed from the dorsal mesoderm. Identify all the parts previously 



