22 INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 



will give rise to the entire central nervous system. It closes over dorsally 

 to form the neural and the brain cavities. These continuous cavities 

 are later almost obliterated by the growth and expansion of their walls 

 to form abundant nervous tissue. Extensions of this central nervous 

 system grow out into all parts of the body and all organs as nerves. 



Dorsal lip 



v,/ 



Region of hindlimb onlage 



Anus 



Early development of the frog embryo. 

 Ventral view of the 1 1 mm. larva ( 12 days' 

 development at 25° C). 



This neurula stage then develops surface ciliation and acquires the 

 characteristic shape of a tadpole having a head, body, and extending 

 tail. 



The embryo is designated as that stage of the frog development be- 

 fore the appearance of a mouth or external gills. There appears a 

 pair of oral sucker-like structures on the head. Shortly after the em- 

 bryo emerges from its jelly capsule (i.e., hatches), it develops external 

 gills for respiration, and is then known as a larva or tadpole. Em- 

 bryonic studies include the tadpole because, until metamorphosis is 



