558 
ZOOLOGY: C. R. STOCKARD 
No blood vessels, or very few if any, form on the extreme anterior 
portion of the yolk-sac, so that the venous end of the heart is never con- 
nected with veins. And the heart does not draw fluid into its cavity 
to be pumped away through the aorta. When the heart cavity does 
contain fluid it is unable to escape and small floating particles may often 
be observed rising and falling with the feeble pulsations. 
3. The hearts in embryos without a circulation are lined with a defi- 
nite endocardium, but the myocardium or muscle wall is poorly devel- 
oped, sometimes consisting of only a single cell layer. Pigmented cells 
are not present in the wall of the normal heart, but in the experimental 
hearts these large chromatophores are invariably found. The cavity 
in many of the hearts is almost if not entirely obHterated by the pres- 
ence of periblastic material and huge amorphous periblast nuclei. 
The conus end of such hearts leads directly to a more or less closed 
ventral aorta. Portions of the aortic arches are seen as open spaces, 
and the dorsal aortae are almost invariably seen as typical spaces lined 
by characteristic embryonic endothelium. 
A point of much importance is the fact, that neither these hearts with 
their endothelial linings nor any portion oj the aortae at any stage of devel- 
opment have ever been seen to contain any form of red blood corpuscle. 
Cells of this type are completely absent from the anterior regions of the 
embryo. 
4. Pigment cells normally occur on the Fundulus yolk-sac and arrange 
themselves along the vascular net so as to map out the yolk-sac vessels 
in a striking manner. Loeb has thought that this arrangement along 
the vessel walls was possibly due to the presence of oxygen carried by the 
corpuscles within the vessels. In the embryos without a yolk-sac circu- 
lation the pigment cells arise but rarely become fully expanded so that 
the usual long branched processes are represented only by short pro- 
jections; the chromatophore consequently seems much smaller than 
usual. 
The unexpanded pigment cells, however, wander over the yolk-sac 
and collect in numbers around the plasma filled spaces. The yolk 
surface of the pericardium and the periphery of the Kupffer's vesicle 
are often almost covered with pigment. The hearts are during early 
stages full of plasma and the pigment cells form a sheath around them, 
while such cells are never present on the normal hearts during the 
embryonic period. 
These facts would seem to indicate that the plasma rather than the 
blood corpuscles contains the substance which attracts the chromato- 
phores and initiates their arrangement along the normal vascular net 
of the yolk-sac. 
