DEVELOPMENT OF THE HEAET. 



395 



of gill-arches, and lies, therefore, between the first gill-arch 

 (k) on the outside, and the anterior intestine (d) on the 



Fig. 147. — Diagrammatic 

 transverse section through 

 the head of an embryonic 

 Mammal : h, horn-plate ; m, 

 medullary tube (brain-blad= 

 der) ; rar, wall of the latter ; 

 7, leather-plate; s, rudiment- 

 ary skull; ch, notochord; 

 ~k, gill-arch ; mp, muscle, 

 plate ; c, heart-cavity, an- 

 terior part of the body- 

 cavity (cceloma) ; d, in- 

 testinal tube ; del, intes- 

 tinal-glandular layer ; df, 

 intestinal-muscle plate ; Jig, 

 heart -mesentery ; Jiw, heart- 

 wall; hk, ventricle; ah, 

 aorta-arches ; a, transverse 

 section through the aorta. 



inside,— just as these vascular arches are situated in adult 

 fishes throughout life. The single main aorta, which results 

 from the union above of these two first vascular arches, 

 soon again divides into two parallel branches, which pass 

 backward on both sides of the notochord. These are the 

 primitive aortse, which have been already spoken of; they 

 are also called posterior vertebral arteries (arterice verte- 

 brates posterior es). These two main arteries send out on 

 each side from four to five branches at right angles, which 

 pass from the body of the embryo into the germ-area, and 

 are called the omphalic-mesenteric arteries {arterice omphalo- 

 mesentericaz), or the yelk-arteries (arterial vitellince). 

 They represent the first rudiments of a circulation within 

 the germ-area. The first blood-vessels, therefore, pass out 



from the body of the embryo and extend to the edge of 



28 " 



