DEVELOPMENT OF THE HEART. 



395 



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

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



Tig. 147. — Diagrammatic 

 transverse section through 

 the head of an embryonic 

 Mammal ; h, horn -plate ; m, 

 medullary tube (brain-blad- 

 der) ; mr, wall of the latter ; 

 1, leather-plate; s, rudiment- 

 ary skull ; ch, notochord ; 

 h, gill-arch ; mp, muscle, 

 plate ; c, heart -cavity, an- 

 terior part of the body- 

 cavity (coeloma) ; d, in- 

 testinal tube ; dd, intes- 

 tinal-glandular layer ; df, 

 intestinal -muscle plate; hg, 

 heart-mesentery; /ay, heart- 

 wall ; hlc, 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 {avtericB verte- 



hrales '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- 



TYiesentericce), or the yelk-arteries (arterice vitellinoi). 



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 



