THE ORGANS OF THE OUTER GERM-LAYER 



491 



(a) The Development of the Otocyst into the Labyrinth. 



The membranous labyrinth is preeminently a product of the outer 

 germ-layer. However great its complication in the adult is, a 

 complication that has given it the name labyrinth, its earliest 

 fundament is exceedingly simple. It arises on the dorsal surface of 

 the embryo in the region of the medulla oblongata (fig. 263 gb), above 

 the first visceral cleft and the attachment of the second visceral arch 

 (fig. 274 above the numeral 3). Here over a circular territory the 

 outer germ-layer becomes thickened and soon sinks down into an 

 auditory pit. This process can be traced very easily in the embryo 

 Chick on and after the end of the second day of incubation, and 

 in the embryo Rabbit fifteen 

 days old. The auditory nerve 

 makes its way from the brain, 

 near at hand, to the fundus 

 of the pit, where it terminates 

 in a ganglionic enlargement. 



The Bony Fishes alone ex- 

 hibit a deviation from these 

 conditions. Just as the central 

 nervous system was in their 

 case formed not as a tube, but 

 as a solid body, and the eye 

 not as a vesicle, but as an 

 epithelial ball, so we see here 

 that instead of an auditory 

 pit there is formed by means 



of the proliferation of the outer germ-layer a solid epithelial plug. 

 This, like the brain-tube and the eye-vesicle, acquires an internal 

 chamber at a later period only namely, after being constricted off. 



The next stage shows the pit converted into an auditory vesicle. 

 In the Chick this takes place in the course of the third day. The 

 invagination of the outer germ-layer grows deeper and deeper, and 

 by the approximation of its margins becomes pear-shaped ; soon the 

 connection with the outer germ-layer becomes entirely lost, as is shown 

 by a section through the head of an embryo Sheep (fig. 275 Ib). 



In nearly all Vertebrates the auditory vesicle is constricted off 

 from the ectoderm in the same manner. The Selachians are an 

 exception : here the auditory vesicle which is metamorphosed into the 

 labyrinth retains permanently its connection with the surface of the 



Fig. 274. Head of a human embryo 7'5 mm. long, 

 neck measurement, From His, " Menschliche- 

 Embryonen." 



The auditory vesicle lies above the first visceral 

 cleft. In the circumference of the visceral 

 cleft there are to be seen six elevations, de- 

 signated by numerals, from which the external 

 ear is developed. 



