STRUCTURE OF THE EMBRYO AT HATCHING 



135 



Thus the hypophysis of the embryo forms only the anterior lobe of the adult 

 pituitary. Its function is extensive, and several hormones — including a growth- 

 promoting hormone and two gonadotropic hormones — have been isolated 

 and purified. 



On each side of the hypophysis are two blood vessels called the dorsal 

 aortic roots. These vessels run forward as the internal carotid arteries and 

 fuse posteriorly to form the dorsal aorta. In the early embryo the dorsal 

 aortic roots connect with the ventral aortic roots by means of the aortic 

 arches (see Fig. 78). 



Ventral to the hypophysis lies the pharynx, lined with endoderm. From 

 the floor of the pharynx an outgrowth (oral evagination) is directed toward 

 an ingrowth of the epidermis (stomodeum) . These two pockets fuse, and an 

 opening forms the mouth. The oral suckers are purely embryonic and larval 

 structures; they degenerate soon after the larva begins to feed. 



ANTERIOR CARDINAL VEIN 



DORSAL AORTA 



EFFERENT ARTERY 



RHOMBENCEPHALON 



ENDOLYMPHATIC DUCT 



NERVE GANGLION 



AORTIC ARC 



AFFERENT ARTERY 



VENTRAL AORTA 



PERICARDIUM 



HEART 



Fig. 78. The structures present in a cross section through the otocysts. The 

 heart and blood vessels are shown in schematic form, since actual sections do not 

 show how the blood circulates from the heart through the external gills. 



