CARDIAC-LOOP FORMATION IN CHICK 375 
Camera-lucida diagrams of the cephalic and cardiac regions 
were made from the whole-mount series. In these diagrams 
the heart and main afferent and efferent vessels, as far as they 
could be made out, were drawn in directly. Later in the work, 
the outlines of the heart and main vessels were completed! from 
dissections and reconstructions of embryos of corresponding 
stages. These diagrams appear as the text figures and serve 
to show at the same time the stages worked on and the relations 
of the heart to the neighboring structures in the body of the 
embryo. 
It was found that the configuration of the heart itself could be 
worked out very successfully from dissections made in alcohol 
under a binocular microscope. In such preparations the heart 
shape is beautifully shown by strong reflected light and can be 
accurately reproduced with the aid of a camera lucida. Em- 
ploying this method, drawings of the same heart were made from 
three aspects to the same scale of magnification. By using 
dividers to keep the dimensions accurate, it was a relatively 
simple matter to make a preliminary clay model of the heart 
from the drawings. This model, with its basic dimensions cor- 
rect, was then finished directly from dissections of the heart, 
which could be rotated and thus studied under the binocular 
microscope from all angles. 
Although the contours are less likely to be distorted in dis- 
sected hearts than in reconstructions, there are certain details 
that cannot be made out satisfactorily by the dissection method. 
The chief point of difficulty is the region of the sinus venosus in 
the older embryos. The manner in which the veins entering 
the heart are imbedded in the surrounding structures renders it 
1 Although a consideration of the changes in the aortic arches does not come 
within the scope of this paper, the condition of the arches at each phase of heart 
development here dealt with is indicated in the text figures. For discussion of 
the development of the aortic arches, reference may be made to the works of 
Boas (’87), Evans (’09), Lillie (’08), and Locy (06). 
The cardinal and umbilical veins are indicated in the figures of the later stages 
only, because in the earlier stages the position of the embryos is such that these 
vessels would have to be superimposed on the heart. Moreover, the early stages 
in the formation of the vessels are shown beautifully in the figures of Evans (’09) 
and Sabin (’17). 
