THE DEVELOPMENT OF THE EMBRYO ITSELF. 99 
presents a curve to the right which represents the venous part 
and one to the left, answering to the arterial. 
‘ of the auricles also are to be seen. 
The rudiments 
The arterial system is represented at this stage by the ex- 
panded portion of the heart known as the bulbus arteriosus, 
and two extensions from it, the aorte, 
which uniting above the alimentary 
canal, form a single posterior or dorsal 
aorta. From these great arterial ves- 
sels the lesser ones arise, and by sub- 
division constitute that great mesh- 
work represented diagrammatically in 
Figs. 112, 113, from which the course of 
the circulation may be gathered. The 
beating of the heart commences be- 
fore the corpuscles have become nu- 
merous, and while the tubular system, 
through which the blood is to be 
driven, is still very incomplete. 
The events of the third day are of 
the nature of the extension of parts 
already marked out rather than the 
formation of entirely new ones. The 
following are the principal changes: 
The bending of the head-end down- 
ward (cranial flexure); the turning of 
the embryo so that it lies on its left 
side; the completion of the vitelline 
circulation ; the increase in the curva- 
ture of the heart and its complexity 
of structure by divisions; the appear- 
ance of additional aortic arches and 
of the cardinal veins; the formation 
of four visceral clefts and five vis- 
ceral arches; a series of progressive 
changes in the organs of the special 
senses, such as the formation of the 
lens of the eye and a secondary optic 
vesicle; the closing in of the optic 
vesicle; and the formation of the na- 
sal pits. 
Fie.  eeserien Bs 
onic vascular system (Wieder- 
sheim). a, atrium; A. A, dor- 
sal aorta ; Ab, branchial ves- 
sels; Acd, caudal artery ; All, 
‘am of the embry- 
allantoic (hypogastric) arter- 
ies ; Am, vitelline arteries; B, 
bulbus arteriosus ; ¢, c,’ exter- 
nal and internal carotids; D, 
ductus Cuvieri (precaval veins) ; 
E, external iliac arteries ; H. C, 
posterior cardinal vein ; Ic, 
common iliac arteries; K. L, 
gill clefts; R.A, right and left 
roots of the aorta; S. S’, 
branchial collecting trunks or 
veins ; Sb, subclavian artery ; 
Sb’, subclavian vein ; Si, sinus 
venosus ; V, ventricle; VC, an- 
terior cardinal vein ; Vm, vitel- 
line veins. 
In the region of the future brain, the vesicles of the 
cerebral hemispheres become distinct; the hind-brain separates 
into cerebellum and medulla oblongata; the nerves, both cra- 
