GILL-ARCHES AND GILL-OPENINGS. 357 



the side of the gullet. In the higher Vertebrates they after- 

 wards close. The gill-arches are transformed partly into the 

 jaws, partly into the tongue-bone and the bonelets of the 

 ear (ossicula auditus). (Of. Plates I., VI., and VII.) 



Almost simultaneously with the development of the gill- 

 arches, and immediately behind these, the heart with its four 

 compartments is formed (Fig. 11 8,0 h i &), and above, on the 

 sides of the head, the rudiments of the higher sense-organs 

 appear : iio.se, eye, and ear. These highly important organs 



FtG. 119. Transverse section through the shoulder region and the front 

 limbs (wing-rudiments) of a Chick, on the fourth day of incubation (about 

 20 times the natural size). Near the intestinal tube three lighter cords are 

 visible on each side in the dark dorsal wall, which pass into the rudiments of 

 the fore limbs or wings (e). The upper of these is the muscle-plate, the 

 middle is the hind, and the lower is the front root of a spinal nerve. In 

 the middle, below the chorda, is the aorta, and on each side of this a cardinal 

 vein ; below the latter are the primitive kidneys. The intestine is almost 

 closed. The ventral wall extends into the amnion, which forms a closed 

 covering round the embryo. (After Remak.) 



