THE DEVELOPMENT OF THE HEART IN MAN. 
261 
a space containing a loose reticulum of tissue, whose features have been worked out 
in similar specimens by Mall (4). 
The endothelial tube containing the blood stream is a very narrow channel indeed, 
lying centrally within the lumen of the myocardial wall. 
The sub-endocardial reticulum shows no division into separate cushions. It 
extends uniformly round the lumen of the tube, and is continuous with a similar 
reticulum in the upper part of the ventricle and in the bulbus. 
The next portion of the heart, from the atrial canal to the root of the truncus 
arteriosus, is divided into two segments, separated from one another by a deep groove 
which traverses obliquely the ventral surface from the left to the caudal right margin, 
and is produced by the acute flexion of the heart tube (Plate-fig. l). 
The homologies of these portions and the successive development of each will be 
traced later ; but for the present the proximal limb may be termed the ventricle, 
while the distal segment is termed the “ bulbus cordis.” 
The ventricle is of the shape of an inverted triangle, the apex forming the apex 
of the heart. Its muscle wall, of considerably greater thickness than that of the 
atrium, is reticulated and separated in the greater part from the endothelial tube 
within by a space containing fine fibrils. Only at the caudally-placed apex are 
these two walls, myocardium and endocardium, in apposition. The atrial canal opens 
at the oral left angle, while the exit to the bulbus is at the corresponding oral and 
right corner. 
At the junction of bulbus and ventricle the lumen of the heart tube is narrowed, 
and a short surface constriction is formed, partially separating them. 
From the interior, this separation of bulbus and ventricle is seen to be due to the 
infolding by the bulbo-ventricular groove of the ventral wall, so that a prominent 
ridge is formed on the ventral and oral aspect extending to the caudal border of 
the lumen. 
The bulbus cordis possesses a myocardial wall of some thickness, a delicate 
reticulum, and in the interior a narrow endothelial blood tube. 
As in the atrial canal a reticulum of this nature precedes the formation of endo- 
cardial cushions, so apparently here also the reticulum precedes the formation of 
definite bulbar cushions. 
The reticulum ceases abruptly at the commencement of the truncus arteriosus, 
and the endothelial tube widens out to line the external wall ; and at this point the 
muscle wall of the heart ceases abruptly, and is replaced by fibrous tissue on the 
wall of the truncus arteriosus. 
The bulbus is roughly of a triangular outline, for near its middle the oral right 
wall is considerably distended. It lies in the ventral surface of the atrium. 
If this specimen be compared with the rather younger embryo heart modelled 
by Thompson (2), a close resemblance of the specimens can be noted, and corresponding 
divisions of the heart tube are present. In his specimen, however, while the ventricle 
