THE ORGANS OF THE OUTER GERM-LAYER. 



509 



organs which grow into the body-cavity are invested by the peri- 

 toneum and supported from its walls by the mesenteries. 



With a reduction in the thickness of the tympanic membrane 

 there occurs a condensation of its connective-tissue substance, 

 whereby it is enabled to fulfil its ultimate function as a vibrating 

 membrane. 



A more extended discussion of the development of the auditory 

 ossicles will ba deferred to a subsequent section, which deals with the 

 origin of the skeleton. At present, only a few words further con- 

 cerning the formation of the external ear, which, as has already been 

 stated, is derived from a depression 011 the outer side of the place 

 of closure of the first visceral cleft. Its 

 development has been minutely inves- 

 tigated in the Chick by MOLDENHAUER 

 and in the human embryo by His. As 

 the lateral view of a very young human 

 embryo (fig. 274) shows, the first visceral 

 cleft is surrounded by ridge-like margins, 

 which belong to the first and second 

 visceral arches, and are early divided into 

 six elevations designated by Arabic nu- 

 merals. From these is derived the auricle, 

 which therefore involves a rather exten- 

 sive tract of the embryonic head (the 

 pars auricularis). The pocket between 

 the ridges, at the bottom of which the 

 tympanic membrane is met with, becomes 

 the external meatus. This is continually 

 growing deeper owing to the surrounding 

 wall of the side of the face becoming greatly thickened ; finally it 

 is developed into a long canal, the wall of which is in part bony, 

 in part cartilaginous. The six elevations mentioned, which sur- 

 round the orifice of the external meatus, together constitute a 

 bulky ring. The accompanying representation (fig. 285) shows 

 clearly its metamorphosis into the external ear. It shows that 

 out of the elevations 1 and 5 the tragus and antitragus are 

 developed, Out of 2 and 3 the helix, and out of 4 the antihelix. 

 The lobule of the ear remains for a long time small ; it is not 

 until the fifth month that it becomes more distinct. It is derived 

 from the hillock marked with the numeral 6. At the close of the 

 second month all the essential parts of the external ear are easily 



Tig. 285. 'Fundament of the 

 outer ear of a human embryo, 

 after His. 



The elevation marked 1 produces 

 the tragus ; 5, the antitragus. 

 The elevations 2 and 3 produce 

 the helix; 4, the antihelix. 

 From the tract 6 is formed the 

 lobule. K, Lower jaw. 



