176 THE GERM LAYER DERIVATIVES 



Neurocoel 



Endolymphatic duct 

 Anterior canal 

 Cartilage 



utricle 



Notochord Pharynx cartilage 



Auditory apparatus of an 11 mm. frog tadpole. 



invaginates to form a vesicle with but a temporary external opening at 

 the 3 mm. stage. This vesicle is the otocyst or auditory sac. After the 

 head ectoderm closes over this invaginating vesicle at the 4 mm. 

 stage, and the walls become continuous, there develops from its dorso- 

 mesial wall a small evagination. This soon (at the 7 mm. stage) be- 

 comes tubular and is known as the endolymphatic duct. The duct on 

 each side grows dorsally to fuse with the duct of the other side of the 

 brain. It finally loses its cavity and forms a vascular membrane which 

 covers the rhombencephalon, having no auditory function whatever. 

 The duct remains as a vestige even in the adult frog, originating be- 

 tween the membranous labyrinth (inner ear) and the hindbrain. 



At about the 11 to 1 2 mm. stage there develops a vertical fold which 

 divides the main cavity of the otocyst into mesial and lateral chambers. 

 The more dorsal and mesial portion is the utricle and the more ventral 

 and lateral portion is the saccule, both being joined by a small pore. 



The utricle, by the 15 mm. stage, becomes further subdivided by 

 three folds or ridges. These develop on the inside of the utricular wall. 

 One is vertical and anterior (anterior semi-circular canal), one is 

 horizontal and lateral (horizontal semi-circular canal), and finally 

 there will appear a third fold which is vertical and posterior (posterior 

 semi-circular canal). These ridges fuse with one another to form 

 three loop-like tubes, each of which opens at both ends into the utric- 

 ular cavity with which it retains connection throughout the life of the 

 frog. These are known collectively as the semi-circular canals which 

 later become free from the utricular wall and continue to grow and 



