376 BRADLEY M. PATTEN 
almost impossible to make clean dissections of this region. In 
the older embryos, therefore, the heart and the main afferent 
and efferent vessels were reconstructed from serial sections by 
the wax-plate method of Born. As far as the principal contours 
of the heart are concerned, these reconstructions were found to 
conform with the clay models made from dissections. They 
furnished, moreover, detailed information concerning the sinus 
region and the entering veins, which it had not been found possible 
to obtain by means of dissections. 
The drawings of the heart shown in the plates were made for 
the most part directly from dissections. They contain some 
details, however, that were added from the wax-plate reconstruc- 
tions. The orientation of the heart in the body of the embryo, 
and the relations of the vessels are shown in the text figures, 
which are lettered in correspondence with the plates. 
THE FORMATION OF THE CARDIAC LOOP 
The youngest stage studied is represented by embryos of 9 
somites (approximately twenty-nine hours’ incubation), in’ 
which the heart is a nearly straight tube (fig. 1, A, and pls. 1, 2, 
and 3, A). Even when the myo-epicardial folds are first approxi- 
mated to each other to form the outer wall of the heart tube, 
there is already a tendency for the right lateral margin of the 
heart to show a greater convexity than that of the left. This 
asymmetry is due, at first, more to unequal dilation of the heart 
wall than to actual bending of the entire tube, as is indicated by 
the fact that the line of attachment of the dorsal mesocardium . 
lies very nearly in the sagittal plane of the body. 
The dorsal mesocardium at this stage forms an unbroken sup- 
porting membrane throughout the entire length of the heart. In 
contrast to the condition in mammals described by Yoshinaga 
(21), the ventral mesocardium in the chick is complete, or nearly 
so, when it is first formed by the approximation of the two folds 
of splanchnic mesoderm which constitute the medial wall of the 
cephalic portion of the right and left coelomic chambers. The 
ventral mesocardium is, however, a more transitory structure 
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