210 PROFESSOR MARSHALL AND EDWARD J. BLES. 



pericardial cavity in the median plane. The first of the two bends 

 forms the auricular portion of the heart, H A, and the second the 

 ventricular, H V. The ventricle lies at a level slightly ventral to the 

 auricle, and is separated from the truncus arteriosus, in front, by a 

 sharply marked constriction. 



Outside the pericardial cavity the truncus arteriosus divides at 

 once into right and left branches, which are much wider than 

 at the earlier stage, and run directly outwards, beneath the 

 pharynx. Each branch gives off from its anterior border a small 

 blind diverticulum into the hyoid arch (Fig. 6, A H), and then 

 divides into afferent trunks for the first and second branchial arches 

 (Fig. 6, A F.I, A F.2), which run outwards a short distance in these 

 arches, and end blindly. 



The heart now contains blood corpuscles, which yary greatly in 

 number in different specimens. In some they are absent, or nearly 

 so, while in others they are abundant. They are all spherical in 

 shape, and loaded with yolk granules. In the hepatic veins the 

 blood corpuscles may be seen separating from the walls of the veins. 



4. The Blood Vessels of the Visceral Arches. 



a. The Mandibular Arch. — An extremely small diverticulum from 

 the front part of the dorsal aorta is present at this stage just 

 behind the level of the infundibulum (Fig. 6, E M). Though 

 extremely small, not exceeding in length the diameter of the aorta 

 at its point of origin, we have found this vessel to be constantly 

 present. Its relations are exactly those of the dorsal portions of the 

 hinder efferent vessels, and we believe it should be regarded as the 

 dorsal or aortic portion of an efferent branchial vessel belonging to 

 the mandibular arch. Its later development and its homologies will 

 be considered afterwards. 



In the ventral part of the mandibular arch a lacunar vessel (Fig. 

 6, V M) is present, which communicates below with an irregular 

 longitudinal sinus lying in the floor of the pharynx, dorsal to the 

 sucker. This mandibular vein, V M, exactly corresponds to the 

 ventral part of the efferent vessel of the hyoid arch, V Y, and may 

 reasonably be regarded as part of an efferent vessel belonging to the 

 mandibular arch. 



b. Ihe Hyoid Arch. — The condition of the vessels in the hyoid 



