242 The American Naturalist. [March, 
innermost soon break down, so as to form a tubular cavity; for 
some time it is simple and undivided, extending, however, through 
nearly the whole length of the embryo. No motion of fluid is, 
seen in the héart or vessels until the 38th or 40th hour. When 
the heart, which may be considered analogous at this period to 
the dorsal vessel of the Annelida, first begins. to pulsate, it con- 
tains only colorless fluid mixed with a few globules. Between 
the 40th and s5oth hours a separation in its parts may be 
FIG. 5.—CIRCULATION IN REPTILES.—z2, eyes hs se in pericardium ; ff, right 
and left auricles Pk — ventricle; a, aorta; v, cava; c, smaller circulation; 4, 
er circulat 
FIG. 6.—CIRCULATION IN MA AMMALS AND BIRDs.—A, heart; v, right ventricle; 
v, ro ventricle; c, right auricle; c’, left auricle; a, aorta ;* d, vena cava; é, greater r cir- 
on; f, pulmonary artery; g, pulmonary veins 
observed, which is effected by a constriction round the middle 
of the tube; and the dilation of the posterior portion becomes an 
auricular sac, and that of the anterior a ventricular cavity. Between 
the 5oth and 6oth hours the tube of the heart becomes more and 
more bent together until it is doubled, so that this organ becomes 
much shorter relatively to the dimensions of the body, and is more 
confined to the portion of the trunk to which it is subsequently 
restricted. About the same time the texture of the auricle differs 
