544 THE HUMAN EMBRYO. 



In the later stages the cartilaginous periotic capsule becomes 

 replaced by bone. This is in chief part spongy bone, but is 

 lined on the surface towards the labyrinth by layers of compact 

 bone, formed by the periosteal membrane. The modiolus and 

 septa of the cochlea, as well as the osseous spiral lamina, are 

 formed wholly in connective tissue, without any preformation in 

 cartilage. 



The Accessory Auditory Organs. 



The Eustachian tube and tympanic cavity are formed from the 

 hyomandibulav pouch, or diverticulum from the pharynx. The 

 pouch does not quite reach the surface at any stage of develop- 

 ment : the membrane closing it at its outer end becomes the 

 tympanic membrane ; and the groove or depression on tho 

 sarface of the head, opposite the hyomandibular pouch, becomes, 

 as already noticed, the external auditory meatus ; the external 

 ear, or pinna, being formed from a series of processes deve- 

 loped round its margin (vide pp. 499, 500). 



Up to the time of birth, the Eustachian tube, and the tym- 

 panic cavity itself in great part, are practically obliterated ; their 

 walls being brought into contact with each other by the deve- 

 lopment of very abundant, gelatinous, connective tissue in their 

 substance. This is absorbed about the time of' birth, and tho 

 tympanic cavity and Eustachian tube opened out. 



DEVELOPMENT OP THE DIGESTIVE SYSTEM. 



1. General Account. 



The alimentary canal of the human embryo, like that of the 

 chick, is at first merely the part of the cavity of the yolk-sac 

 which is included within the embryo, as this is constricted off by 

 the head-, side-, and tail-folds (cf. Fig. 188). 



As the constriction deepens, to form the yolk-stalk, the part 

 of the cavity within the embryo, or mesenteron, gradually be- 

 comes more and more sharply separated from the cavity of the 

 yolk-sac proper ; the two cavities still, however, communicating 

 freely through the yolk-stalk. 



The mesenteron soon acquires a definite tubular form, and 

 on the fifteenth day has the shape and relations shown in Fig. 2'i2. 



