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 12s 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: 
