520 Proceedings of the Royal Society 
vascular chain of sinuses along the ventral surface of the serous or 
animal lamina of the embryonic germ disc, which, being suddenly 
arrested by a large projection of the germ vesicle at the anterior 
part beyond the vascular and mucous lamina, causes a stagnation in 
the form of a conical sac, whose apex points forward below the 
head of the embryo at that time consisting of the mesocrane and 
procrane, with the visceral segments of the face-kist, and exhibiting 
three facial clefts. 
1. The ophthalmic, under the procrane, forming the roof of the 
orbit, and completes its floor resting on the antrum highmorianum. 
2. The olfactory cleft, the nasal passage beneath the antrum, the 
ethnoid and turbinated bones, its floor being the hard palate. 
3. The oral cleft, below the palate, forming the roof of the mouth, 
while the hyoid and tongue form the floor, and the maxilla and 
muscles of the cheek complete the cavity of the mouth. 
I. As soon as the conical sac of the venous sinuses and azygeal 
veins from the chorda centralis(the future spinal column) ' extend 
to the upper angle of the first formed embryonic heart, where two 
tubes or inferent veins enter from the lower angle of its base, a single 
efferent tube — the ductus venosus — leading backwards, and sending 
forwards from the umbilicus a vein on each side (in birds towards 
the circular sinus, from which a fringe of minute capillaries extend 
below the chorion). In the capillary area, where the veins and 
arteries communicate, the venous blood becomes oxidised, and re- 
turns by the arterial area to the embryo; the minute arterial rootlets 
becoming larger and larger, till the blastine artery from each side 
enter the lower part of the embryo above the highly upturned coccyx, 
the lower orifice of the future umbilicus, when the abdomen is closed. 
From this point these arteries (the future umbilical of the foetus) 
descend to the iliacs, which unite opposite the promontory of the 
sacrum, and carry the arterial blood upward along the chorda cen- 
tralis, as high as the fovea cardiaca, or pharyngeal hollow, from 
whence the double aortse each divide into two terminal branches, 
which unite behind the base of the heart to form the arterial aortic 
bulb, quite unconnected with the cavity of the heart, the top of the 
bulb seen rising higher than the heart, has misled embryologists to 
fancy it was a part of a twisted tube, which has sometimes been 
described as the first form of the embryonic heart. Pander, Prevost, 
