EMBRYOLOGY OF TADPOLE AND CHICK 549 



animal debris, and in the laboratory will thrive on a diet of cereals. As 

 the egg is dependent upon temperature for its rate of speed in develop- 

 ing, so the rate of speed at which the tadpole grows is dependent upon the 

 quantity of food it obtains. 



External gills develop shortly after hatching, being used as respira- 

 tory organs. These disappear as soon as the mouth opens, and the true 

 internal gills are formed. When the true gills form, they are protected 



by a cover called the operculum. The por- 

 tion underneath the operculum remains con- 

 nected with the outside by only a single pore 

 on the left side, known as a spiracle. The 

 limb-buds appear normally at about four or 

 five weeks, although in the laboratory at a 

 1 rig. sis. higher temperature, much sooner. The an- 



i, Front view of the mouth of a tcrior pair develop within the opcrcular 



tadpole of Rana temporaria show- ., , , 



ing the transverse rows of tiny cavity, and consequently cannot be seen 

 horny *& \i^T SS&& from the outside. The posterior develop, 



(After Gutzeit.) Qne Qn each s j de Q the c l oaca a li tt l e l ate r, 



and become quite large and jointed by the end of the second month. 



In the meantime the lungs have been growing and the young tad- 

 pole comes to the surface of the water to expel small bubbles of air and 

 to take in a fresh supply. In the common species of frogs, metamor- 

 phosis begins at about the end of the third month. It is at this time 

 that the tadpole ceases feeding, and the outer layer of skin, as well as 

 the horny jaws (Fig. 318), are thrown off. The lips shrink, the mouth 

 is no longer suctoreal, and becomes much wider, while the tongue in- 

 creases in size. The eyes also become prominent. The fore-limbs ap- 

 pear, the left one pushing through the opening of the gill chamber, while 

 the right pushes its way through the opercular fold on that side, leaving 

 a ragged hole. The stomach and liver enlarge, while the intestine be- 

 comes shorter and smaller in diameter than before, and the animal be- 

 comes carnivorous. The gill-clefts close and many changes occur in the 

 blood vessels due to the change in the animal's mode of breathing. The 

 bladder is formed, the kidneys undergo changes, and there is a definite 

 sexual differentiation. The tail shortens and is finally lost as the hind 

 legs continue to lengthen. 



If, however, the water has been particularly cold, the metamor- 

 phosis may be put off until the following spring ; in fact, it seems normal 

 with some species to wait even -longer than this, namely, as long as two 

 years, and sometimes three (Necturus) before the adult form is assumed. 



If one thinks of the hen's egg being laid without the normal shell, 

 such a shell-less egg would be somewhat akin to the frog's egg. Here 

 the yolk is a rather blackened mass with a jelly-like substance surround- 

 ing it, similar to the white of the hen's egg, but without a solid shell. 

 Great masses of the eggs are found in one place, appearing very much 



