CHAPTER TEN 



A Survey or tlie Later Embrvo or Larva 



External Features 

 Metamorphosis 



External Features. 



The external changes in shape of the frog embryo and early larva 

 are continuous. The body is elongated and a posteriorly directed tail 

 bud and tail develop, just dorsal to the original position of the blasto- 

 pore. This tail carries with it an extension of the notochord, myo- 

 tomes, and blood vessels as well as the pigmented epidermis of the 

 body. It shows contractions of the < -shaped muscle blocks even 

 before the time of hatching. 



The previously described surface changes are further accentuated. 

 The yolk mass accounts for the ventral bulge in the belly region. 

 Anteriorly the pronephric and gill bulges are more prominent. The 

 olfactory pits appear in the original placodes, dorsal and slightly 

 lateral to the stomodeum. All four branchial grooves are developed 

 and the rudiments of the external gills are beginning to grow from 

 the upper levels of the first and second branchial arches. 



The embryo hatches when it measures about 6 mm. in total length. 

 Since no food is ingested during this period, the interval between 

 the fertilization of the egg and hatching depends almost entirely upon 

 the temperature of the environment during development. The hatch- 

 ing process probably is accomplished by the aid of temporary glands 

 in the sucker region. These glands presumably elaborate an enzyme 

 which aids in digesting away the surrounding jelly coverings, allowing 

 the larva to escape. The jelly itself is not a food for the larvae, or 

 tadpoles, even though after hatching they often are seen attached 

 temporarily to the empty jelly capsules. 



The pre-hatching embryos show constant swimming movements 

 due to the presence of surface cilia. At this stage, and immediately 

 after hatching, there is also considerable muscular movement and 

 the entire larva (tadpole) may show occasional coil and S-shaped 

 body contractions which are entirely muscular. 



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