DEVELOPMENT OF THE FROG 1 (2) 



4. Examine, in a watch glass of water as before, a later stage in the 

 development of the Frog in which the body of the animal has begun to 

 elongate in an anteroposterior direction. Note that the blastopore, 

 which was circular, has now become a slit-like opening which marks the 

 posterior end of the embryo. Note also the dorsal, longitudinal groove 

 (neural groove) which indicates the position of the future central 

 nervous system. Draw the embryo from the posterior end. 



5. Examine, as before, still later developmental stages as follows: 

 (a) an embryo in which the head and tail have just become differenti- 

 ated; (6) an embryo with well-developed external gills; (c) a fully 

 formed, free-swimming tadpole, in which the external gills have become 

 covered by the operculum, leaving only a small opening (spiracle) on 

 the left side of the animal in the gill region ; (d) an embryo in an early 

 stage of metamorphosis, showing a single pair of small hind legs, and 

 (e) an embryo in a late stage of metamorphosis, with both pairs of legs 

 present and the tail in process of absorption. Draw each of these em- 

 bryos from the left side and label in each stage, where developed : nose, 



EYE, MOUTH, SUCKER, GILLS, OPERCULUM, MUSCLE PLATES, BLASTOPORE, 



anus, and tail. 



» B. pp. 225-232. 



361 



