CIRCULATORY SYSTEM 777 



mesocardium remaining- which connects the apex of the heart to the 

 pericardial wall. The walls of this tube are now called the myoepicardial 

 mantle and the vascular cells which are enclosed within this mantle are 

 those which form a continuous sheet and become the endocardium or 

 lining of the heart. 



There are still some vascular cells cephalad and caudad to this tubu- 

 lar heart. These furnish a lining for the blood vessels which arise from 

 the edges of the lateral plates and connect with the heart. The first ves- 

 sels toward the head end (the anterior pair) become the mandibular 

 arteries, while those vessels lying caudad to the heart (the posterior) 

 are called the omphalo-mesenteric veins. 



It is at this time also that immediately cephalad to the omphalo- 

 mesenteric vein a transverse tube appears on each side connecting with 

 the heart tube, and it is these tubes which are the ducts of Cuvier. The 

 ridge where the Cuvierian ducts grow, becomes larger until it forms 

 a transverse partition known as the septum transversum (Fig. 348). 

 It is this septum or partition which separates the heart cavity or peri- 

 cardial region from the abdominal or peritoneal cavity. In the myxinoids 

 and elasmobranchs this septum never completely closes dorsally, but 

 leaves one or two openings known as the pericardia-peritoneal canal. 



Where the early embryo is closely appressed to the very large yolk 

 sac, as in the bony fishes and in all amniotes, the development of the 

 heart is modified. The pharynx is not complete below at first, but com- 

 municates ventrally with the yolk. The two hypomeres are thus pre- 

 vented, for a time, from meeting ventrally. "Each, however, is accom- 

 panied by its vascular cells ; its edge becomes grooved and the grooves 

 are rolled into a pair of tubes, lined with endocardium, so that for a time 

 the anlage of the heart consists of two vessels (Fig. 283), each connected 

 in front and behind with its own mandibular artery and omphalo-mesen- 

 teric vein, and is surrounded with its pericardial sac. Later the two 

 tubes approach and fuse with the formation of mesocardia as before : 

 these latter soon disappear, leaving the whole much as in the small- 

 yolked forms." 



While the pericardium is relatively large at first, in adult forms it 

 is usually quite close fitting to the heart when the heart is expanded. 



It must not be forgotten that in systole the heart contracts and be- 

 comes considerably smaller than normal ; that in diastole it expands and 

 attains its full size, filling the pericardium accordingly. 



It can readily be understood that as long as the mesocardia are 

 present the heart tube will be a straight canal connected with the peri- 

 cardial sac in front and behind. However, as the mesocardia entirely 

 disappear in due time and the heart tube continues to grow, it bends upon 

 itself something like a capital letter "S," the bending or flexure being 

 largely in a vertical plane (Fig. 283). 



At the middle point of the bend the tube remains quite small and 



