22 Dynamic Tlieory 



on the inner side of the gill arches after the manner that is permanent 

 in the primitive fishes. But afterwards, in all the higher vertebrates, 

 the anterior two of these disappear and the permanent main arteries de- 

 velop from the posterior three, the lung arteries from the last one. 



Fio. 27. The five arterial arches as originally formed in the skulled vertebrates, a. 

 Arterial stalk, a .Main stem of aorta, c. Head artery (Carotis.) (Eathke;) 



FIG. 28. The arterial arches as formed in birds the unshaded parts are rudimentary 

 and disappear the dark parts remain, a. Arterial stalk, a". Main stem of aorta. 

 c. Carotis. p. Lung artery, p'. Branches of same, .s. Subclavian arteries. (Rathke) 



FIG. 29. The five arterial arches of mammals. The light parts disappear letters as in 

 last fig. v. Vertebral artery. &. Botallis duct used by the embryo but afterwards 

 closed. (Rathke.) 



Directly after the formation of the medullary tube, notochord, and 

 intestinal tube, the heart begins to form from the front end of the intes- 

 tinal tube and in- connection with the blood vessel sj'stem. The first 

 appearance of a heart is in the form of a straight spindle-shaped organ 

 without a cavity. (The blood vessels also first appear as cords without 

 cavity.) Soon, however, a cavity is developed in it, and it takes a 

 shape approximating that of the letter s. From each end of it two 

 blood vessels proceed. Those in front passing by the first gill arches, 

 one on each side, after the manner that is permanent in adult fishes, 

 unite above the intestine and run thence backward as a single aorta just 

 below the notochord. It soon divides and sends ramifications first all 

 over the germ area and then into the yelk sac. The two vessels from 

 the posterior end of the heart are veins and bring the blood back to the 

 heart. The heart itself next divides into two cavities, an auricle and a 

 ventricle. Leading up from the ventricle is a tube from which the 

 aortal arches spring, and leading into the auricle are the two veins. 

 The auricle has in the rear, on each side, an appendage called the auri- 

 cular process. This is permanent in the lower fishes, and the whole 

 heart, at this stage, is the heart of an adult fish. 



Later, the auricle divides into two chambers, and lastly, the ventricle 

 also divides into two chambers. In the human embryo this stage is 

 i^o oi, o,i- hv tlift fourth week of embryo life. The lungs, in the mean- 



