356 THE EVOLUTION OF MAX. 



cavity grows thicker, and is changed into a bow-shaped or 

 sickle-shaped ridge : these are the gill-arches ; on their 

 inner side a vascular arch afterwards arises (Fig. 101, p. 336). 



1. 



FIGS. 116, 117. Head of a Chick, on the third day of incnbation : 116 is a 

 front view ; 117 from the right side ; n, nose-rudiments ; I, eye-rudiments ; 

 [7, ear-rudiments ; v, front-brain ; gl, eye-fissures. The first of the three 

 pairs of gill-openings is separated into an upper jaw process (c) and a lower 

 jaw process (u). (After Kolliker.) 



Fig. 118. Head of embryo of a dog, from the front : a, the two side halves 

 of the front brain-bladder ; b, eye-rudiments; c, middle brain-bladder ; de,tho 

 first pair of gill-arches (c, upper jaw process ; d, lower jaw process) ; /, /', f" r 

 the second, third, and fourth pair of gill-arches; g h i k, heart (g, right, li, 

 left auricle ; i, left, fc, right ventricle) ; I, origin of the aorta, with three 

 pairs of aorta-arches, which pass on to the gill-arches. (After Bischoff.) 



The number of the gill-arches, and of the gill-openings, 

 which alternate with the former, amounts in the higher Ver- 

 tebrates to four or five on each side (Fig. 118, e, d,f, f, /'). 

 The lower Vertebrates have a yet larger number. Origin- 

 ally these remarkable formations discharged the function of 

 a breathing-apparatus were gills. Even yet in the Fishes 

 generally, water, serving for respiration, and which is taken 

 in through the mouth, passes out through the gill slits on 



