138 



DEVELOPMENT OF THE HEAKT. 



The sinus venosus may be described as consisting of two lateral enlargements or 

 horns, and of a transverse part connecting these horns. The veins which it at this 

 time receives are the umbilical, the vitelline, and the ducts of Cuvier (formed 



Fig. 165. CONDITION OF THE HEART IN THE HUMAN EMBRYO OP ABOUT FIFTEEN DAYS, RECONSTRUCTED 



FROM SERIAL SECTIONS. (His.) 4 { y 



A, from before, showing external appearance of heart ; B, the same with the muscular substance of 

 heart removed showing the endothelial tube ; C, from behind. 



inn, mandibular arch with maxillary process ; Tiy, hyoidean arch ; b. a, bulbus aortas ; v, right 

 ventricle ; v', left ventricle ; au, auricular part of heart ; c.a., canalis auricularis ; s.r, horn of sinus 

 venosus with umbilical vein (u.v), superior vena cava (v.c.s), and vitelline vein entering it ; 

 d, diaphragm ; m p, mesocardium posterius ; I, liver ; b.d, bile duct. 



by the junction of the primitive jugular from the head and the cardinal from 

 the trunk). The three veins are nearly symmetrical on the two sides, and enter the 



l.au. b.a. 



l.v. 



Fig. 166. HEART OF A SOMEWHAT MORE ADVANCED HUMAN EMBRYO. (His.) -f 

 A, from before ; B, from behind. 



r.v, right ventricle ; l.v, left ventricle ; b.a, bulbus aortse ; r.au, right auricle ; Lau, left auricle ; 

 v.c.s, vena cava superior ; u.v, umbilical vein ; v.v, vitelline vein ; d, diaphragm. 



corresponding horn of the sinus (fig. 168). The sinus is at first in free communi- 

 cation with the common auricular cavity, but the junction presently becomes 

 narrowed, and the resulting aperture, which eventually acquires a slit-like character, 



