THE FROG 43 



4. Blastopore. The pigmented cells divide more rapidly 

 than the white cells and come to grow over the latter; at 

 the same time the pigmented cells become infolded and grow 

 into the egg itself. This continues until but a small mass of 

 white cells can be seen. The term blastopore is applied to 

 the opening, and the mass of white cells is spoken of as the 

 yolk plug. Later the blastopore closes and the yolk plug 

 is forced inside the egg. 



5. Neural Groove. On what is to become the dorsal side 

 of the body will be found an open groove with slightly 

 elevated margins. How far does it extend? Is. it open 

 throughout its length? Examine several specimens. What 

 distinguishes the part which is to form the brain from that 

 which will produce the spinal cord? The margins grow 

 together to form a tube and this subsequently develops into 

 the brain and spinal cord. How does the shape of the 

 embryo compare with the earlier stages? In eggs which 

 have elongated, is there any marked difference between 

 dorsal and ventral, anterior and posterior? 



6. Gill Stage. In a tadpole just hatched observe the be- 

 ginning of the formation of head, body and tail. There is no 

 mouth yet present, but on the ventral side of the head are 

 suckers by which the tadpole adheres temporarily to some 

 support. Nostrils and eyes are beginning to form, and gill 

 buds indicate the position of future gills. In a somewhat 

 later stage finger-like gills are produced upon the sides of 

 the head. At such a period mouth, eyes, nostrils and tail 

 are well formed, and the internal organs are undergoing 

 their development. 



7. Tadpole. Distinguish head, body and tail regions. 

 What has become of the external gills? The fold of skin 

 covering the internal gills is the operculum and the opening 

 to the gill chamber is the spiracle. On which side is the 



