THE DEVELOPMENT OF THE EMBRYO ITSELF. 109 



ward, bends round to the dorsal side of the pharynx, each giving 

 off at right angles to the yelk-sa,6 a vitelline artery ; the aortse 

 unite dorsally, then again separate and become lost in the pos- 

 terior end of the embryo. The so-called arches of the aorta are 

 large branches Ln the anterior end of the embryo derived from 

 the aorta itself. 



The venous system corresponding to the above is composed 

 of anterior and posterior pairs of longitudinal (cardinal) veins, 

 the former (jugular, cardinal) uniting with the posterior to form 

 a common trunk (ductus Cuvieri) by which the venous blood is 

 returned to the heart. The blood from the posterior part of the 

 yelk-sac is collected by the vitelline veins, which terminate in 

 the median sinus venosus. 



The Later Stages of the Fcetal Circulation. — Corresponding 

 to the number of visceral arches five pairs of aortic arches arise ; 

 but they do not exist together, the first two having undergone 

 more or less complete atrophy before the others appear. Figs. 

 115, 116 convey an idea of how the permanent forms (indicated by 

 darker shading) stand related to the entire system of vessels in 

 different groups of animals. Thus, in birds the right (fourth) 

 aortic arch only remains in connection with the aorta, the left 

 forming the subclavian artery, while the reverse occurs in 

 mammals. The fifth arch (pulmonary) always supplies the 

 lungs. 



Fig. 115. — Diagrams of the aortic arches of mammal (Landois and Stirlinff, after 

 Rathke). 1. Arterial trunk with one pair of arches, and an indication where the 

 second and third pairs will develop. 2. Ideal stage of five complete arches: the 

 fourth clefts are shown on the left side. 3. The two anterior pairs of arches have 

 disappeared, 4. Transition to the final stage. A, aortic arch; ad, dorsal aorta; 

 ax, suhclavian or axillary artery; Ce, external carotid; Ci, internal carotid; dB, 

 ductus arteriosus Botalli; P, pulmonary artery; S, subclavian artery; ia, truncus 

 arteriosus; v, vertebral artery. 



The arrangement of the principal vessels in the bird, mam- 

 mal, etc. , is represented on page 110. In mammals the two prim- 



