No. 408.] STUDY OF MAMMALIAN EMBRYOLOGY. 929 
the limb; the nerves are the rami ventrales (R.V.) of spinal 
nerves, and form branches within the limb; one of these 
branches, as yet unidentified, is shown at Vv. The vein Scl V. 
is the so-called subclavian or axillary vein, a branch of the 
jugular, as explained below. The lower part of the section is 
occupied by the large heart lying in the pericardial chamber. 
The body-wall (Som.), or somatopleure, which forms the outer 
covering of this chamber, is quite thin, and without a trace of 
muscular or skeletal structures. The heart consists of two 
auricles and two ventricles. The auricles have thin walls and 
are separated from one another by a very thin membrane, the 
septum superius (S.s.). The right auricle (Az.d.) receives 
upon its dorsal side the opening of the vein or duct of Cuvier, 
this opening being guarded by valves; of these valves, the one 
towards the median line disappears, but the other, towards the 
right of the embryo, persists to form both the Eustachian and 
Thebesian valves of the adult. The corresponding opposite 
vein, or left duct of Cuvier (D.C.S.), is almost symmetrically 
placed, but does not have any communication with the left 
auricle, being instead connected at a lower level by a transverse 
venous trunk with the ductus Cuvieri of the right side. The 
upper portion of the left duct (D.C.S.) is seen in the section to 
be somewhat constricted off from the lower portion, and in fact 
it shows the jugular vein of the head at its actual junction with 
the ductus. The ventricles of the heart are much larger than 
the auricles, and the left ventricle ( Vzz..S.) is already larger 
than the right; the external groove (f.) which marks the 
boundary between the two ventricles, is clearly shown by the 
section. The trabecular structure of the ventricles is well 
developed and affords a diagnostic mark by which the ven- 
tricles, however cut, may be easily recognized in sections. 
The development of the trabeculz corresponds to the forma- 
tion of the blood sinusoids of the heart, to which I have re- 
cently directed attention. The constricted region of the heart, 
which connects the auricles with the ventricles, is known as 
the auricular canal A broad partition (S.a.c.) divides the 
canal into right and left channels, and at the ventricular ends 
of these channels the formation of atrio-ventricular valves 
