120 THE DEVELOPMENT OF THE CHICK 



reach the embryo. The myocardium then becomes arched 

 towards the body-cavity and includes the endocardium in its 

 concavity (Fig. 53). The heart thus comes to consist of two 

 parts on each side: a myocardial gutter semicircular in cross 

 section, open towards the entoderm, and an endothelial tube 

 lying in the gutter, and in contact with the entoderm. At this 

 time the lateral limiting sulci appear in the splanchnopleure 

 just central to the endocardium on each side, and, as the fore- 

 gut closes from in front backwards, the following changes take 

 place (Figs. 54 and 54 A) : (1) the entoderm withdraws completely 

 from the fused apices of the lateral folds in the splanchnopleure, 

 and thus a wide separation is made between the floor of the pharynx 

 and the splanchnopleure below; (2) the right and left endocardial 

 tubes come into immediate contact in the floor of the pharynx; 

 (3) the two myocardial gutters coming together form a single 

 tube around the endocardium, suspended by a double mesoder- 

 mal membrane (mesocardium or dorsal mesentery of the heart) to 

 the floor of the pharynx, and attached by a similar mesentery 

 (ventral mesentery of the heart) to the splanchnopleure beneath 

 (Fig. 54). The latter connection is ruptured almost as soon as 

 formed, so that the floor of the myocardium becomes complete 

 (Fig. 54 A). Soon after the completion of the floor of the phar- 

 ynx the two endocardial tubes press together until the common 

 wall becomes reduced to a vertical partition, which then ruptures; 

 and finally (10-12 s) all traces of the original duplicity of the 

 heart disappear (Figs. 60, 62, 64). 



The heart thus arises from two lateral halves which fuse sec- 

 ondarily to form a single tube. This fusion takes place from 

 in front backwards, hence the anterior end of the heart is formed 

 first. Indeed, the full length of the cardiac tube is not formed 

 in the period covered by this chapter; the definitive hindermost 

 division is established by concrescence after the 12 s stage. But 

 the actual hind end is always continuous with the extra-embryonic 

 network of blood-vessels and this connection develops into the 

 main splanchnic veins. 



As a rare abnormality the lateral primordia of the heart may meet 

 and fuse dorsal to the embryo, instead of in the floor of the pharynx. 

 This condition is known as omphalocephaly ; in other rare cases the lateral 

 halves may fail to unite, and two hearts may be formed. 



There are three views concerning the origin of the endocardium: 



