82 
.lANE I. KOBERTSOX. 
wall of tlie (levelopiDg sinus venosus being still in close 
relationship with the pharyngeal rudiment above. In this 
adjustment of the lieart-tube to the pericardiac space, the 
descending and ascending limbs of the loop, from being pos- 
terior and anterior respectively, come to lie side by side, the 
former on the left and the latter on the right (Text-fig. 8). 
That is to say, the anterior and posterior ends of the heart are 
fixed, but the loop as a whole rotates approximately tlirough 
a right angle in a clockwise direction as seen in a dorsal 
view of the heart, tlius twisting the long axis of that organ 
into a position approximately transverse to, instead of parallel 
with, the axis of the embryo. This position is maintained 
Diagram to illustrate the looping of the heart on itself ; (a) repre- 
sents the primitive and (b) the secondary position of the heart. 
A. Anterior. P. Posterior. 
till the maximum vertical development of the heart has been 
attained, when a degree of untwisting occurs as the pericar- 
diac space begins to increase rapidly in antero-posterior 
depth, and the adult form and position of the heart are 
reached. 
From Stage 25 the heart consists for a time of a double- 
walled tube, the inner wall being endothelial and the outer 
myocardial (Text-fig. 9). 
The tube is bent in a narrow U -shape, its anterior and 
posterior ends being in close proximity to one another, and 
both approximately in the middle line, the former terminating 
on either side in the lateral ventral aorta, the latter forming 
the sinus venosus, somewhat to the right. This double U-tube 
may be divided into four parts : (1) A posterior descending 
auricular part (Text-fig. 9 a, A.), the axis of which forms 
Text-fig. 8. 
A 
B 
