THE EMBRYO 



131 



Fig. 74. Two salamander embryos photographed in side view with the head 

 at the right. Left: The embryo is still within the egg membrane and moves very 

 slowly by means of the cilia of the epidermis. The dorsal surface is still convex, 

 the head is bent ventrally, and the tail bud is very small. Right: The embryo is 

 longer and has straightened up so that the back is becoming flattened. It will 

 contract its muscles when stimulated. The gill region is divided into three parts, 

 each of which is a forerunner of an external gill. The head is lifted up from the 

 abdomen and a nasal pit is visible. Unlike the frog, this embryo has not yet 

 hatched from the egg membrane. (Courtesy Professor John A. Moore, Columbia 

 University.) 



PRONEPHRIC 

 SWELLING 



I GILL 



*4 



v 



GILL 



NASAL PIT 

 SUCKERS 



Fig. 75. A side view and a head view of a Rana sylvatica embryo at the time 

 of hatching (stage 20). (Courtesy Professor Arthur W '. Pollister, Columbia Uni- 

 versity.) 



nizable structures, such as a head as distinct from the body and the beginnings 

 of tail formation. The dorsal surface of the embryo also takes on shape, 

 owing to the formation of the neural tube and the mesodermal somites. 

 After stage 17 the embryo gradually elongates; the head becomes de- 



