8 Vertebrate Embryology 



and as it passes through the anterior, thick- 

 walled part of the oviduct a gelatinous envel- 

 ope is deposited around it. This envelope, on 

 coming in contact with the water, swells enor- 

 mously, and forms a mass of colorless jelly, 

 characteristic of frog's spawn (Fig. 4). The 



Fig. 4. — Various stages in the development of the frog. 

 (After Brehm from Marshall.) 



1. Eggs just laid. 2. Eggs shortly after laying. 3. Tadpole shortly before 

 hatching. 4. Tadpoles just Hatched. 5 and 6. Tadpoles with external gills. 

 7 and 8. Tadpoles with fully-formed opercular folds. 9 and 10. Tadpoles with 

 well-developed hind legs, shortly before the metamorphosis. 11. Tadpole during 

 the metamorphosis. 1§. Young frog with tail only partially absorbed. 



thin, gelatinous envelope is said to begin to 

 swell about one minute after it comes in con- 

 tact with the water, and to reach its greatest 

 expansion in three hours. Its purpose is to 

 protect the soft eggs from being injured by 



