THE ANATOMY OF AN AMPHIBIAN EMBRYO 



71 



the forebrain the epidermal component of the pituitary gland arises and 

 migrates to fuse with the forebrain. 



Opposite the hindbrain is a pear-shaped otocyst, most of which gives 

 rise to the sensory portions of the organ of balance and hearing. The hind- 

 brain merges into the spinal cord, which supplies the innervation for the 

 muscles and skin of the back, the limbs, and. by way of the sympathetic 

 ganglia, one component of the nerves of the viscera. 



Underneath the spinal cord the roof of the archenteron has differentiated 

 into a notochord, which serves as a nucleus for the later differentiation of the 

 vertebrae. On either side of the notochord there is a pronephros, consisting 

 of three pronephric tubules and a pronephric duct. These structures serve as 

 the excretory organs of the embryo. The pronephric duct empties into the 

 hind-gut. The gut extends anteriorly from the anus and forms a large cavity 

 forward, the pharynx. The first indication of the liver is a pocket of the gut 

 near the heart. The heart differentiates from the mesoderm in the vicinity 

 of the pharynx and is first present as two heart fields, which fuse to form the 

 single, tubular heart. The lateral mesoderm forms two layers, the somatic 

 and splanchnic layers of the mesoderm, with a cavity, the coelom. The bulk 

 of the yolk of the egg remains as a large mass which is gradually used up 

 during the development of the embryo. 



A diagram of a cross section of the embryo shows additional structures 

 (Fig. 31). On the dorsal side a section cuts through the spinal cord and 



Fig. 31. Cross section of 

 an embryo. In such a section 

 the structures can be identified 

 by their shape, size, and rela- 

 tive positions. 



SPINAL CORD 



NEPHROTOME 



NEURAL CREST 



PRONEPHRIC 

 DUCT 



SOMATIC 

 MESODERM 



SPLANCHNIC 

 MESODERM 



