Embryology. 21 



reous matter as a core. This core afterwards becomes the tissue accom- 

 panying the central artery of the eye.* 



The ear, in the meantime, is developed in connection with the posterior 

 brain bladder the bladder of the medulla oblongata. The skin oppo- 

 site this part is first bulged out and thickened. It then becomes in- 

 dented from without, the indentation increasing till a cup is formed. 

 This cup is then pinched off from the outside skin, sinks back into the 

 head and becomes a sac of the shape of a pear, and is filled with a fluid. 

 It is connected with the brain bladder by a slender pedicle which after- 

 wards becomes the portio mollis or auditory nerve. The pear shaped 

 sac is divided into two apartments, the posterior of which is the utriculus 

 in which the semi-circular canals are subsequently developed, and the 

 other is the sacculus in w r hich are developed later the cochlea, the stairs, 

 &c. These two sacs together constitute the labyrinth of the ear. From 

 the first gill opening, to be described presently, the ear drum and the 

 eustachian tube leading from the drum to the pharynx, are formed. 

 Lastly the external canal or meatus from the drum through the skull to 

 the outside, and the ear flap are developed. The formation of the bones 

 in the ear drum is also a late development. 



FIG. 26. Head of embryo fish show- 

 ing arrangement of gill arteries. 



dc. Junction of front and hind veins. 



sv. Venous sinus vein receptacle. 



a. Auricle. 



v. Main chamber. 



abr. Stem of the gill arteries. 



s. Gill slits between the arterial 

 arches. 



ad. Aorta suppl y ing purified blood 

 to the body. 



c. Head artery supplying brain, &c. 



n. Nose groove. 



(Gegenbaur.) 

 FIG. 26. 



Now turn back to the development of the forward part of the body as 

 it lies, belly down, upon the yelk sac. In front of the yelk sac the exo- 

 derm, with its skin stratum and flesh stratum, has curved under the 

 medullary tube from each side and coalesced there forming the outer 

 casing of the body. It has, however, left four vertical openings on 

 each side in the skin just under the brain bladders which are called the 

 gill openings. The skin between these openings forms the gill arches. 

 They represent the permanent gills of fish and amphibians. In the 

 mammal embryo they are subsequently diverted and developed into 

 other forms or entirely obliterated. The first gill slit becomes the 

 track of the eustachian tube, tympanum, and meatus, of the ear as be- 

 fore mentioned leading from the mouth through the drum to the outer 

 opening of the ear. 



The first system of arteries includes temporary arterial vessels lying 



* See Hseckel's Evolution, 2592. 



