HEAD-FOLD TO TWELVE SOMITES 119 
and the embryo has been interpreted in two principal ways: 
(1) that they are an ingrowth from the original vascular primor- 
dium of the opaque area; and (2) that they arise by differentia- 
tion in situ. The first view was originally stated by His, and 
has been supported by Ixolliker and others. The second is sup- 
ported by Rickert, P. Mayer and others. The observations, 
on which the ingrowth theory of His were based, were made 
originally on whole blastoderms of the chick, and concerned 
primarily the order of origin of the blood-vessels, which is cen- 
tripetal. But it is obvious that such observations do not in 
themselves demonstrate the existence of an independent ingrow- 
ing primordium; they are not in the least inconsistent with the 
view that the blood-vessels differentiate from the cells im situ. 
Within the embryo itself parts of certain vessels appear to arise 
separately, and form secondary connections with the vessels 
formed at an earlier time; this is the case for instance with the 
dorsal aorta in the region of the head. The histological appear- 
ances accompanying the first origin of blood-vessels in the pel- 
lucid area appear to favor the view that they arise from detached 
cell-groups of the splanchnic mesoderm. It would, however, be 
going too far to assert that the embryonic blood-vessels have no 
independent power of growth; certain appearances cannot be 
satisfactorily explained in any other way. 
Origin of the Heart. The embryonic heart possesses two 
layers: an internal delicate endothelium, the endocardium, and 
an external strong muscular layer, the myocardium. The endo- 
cardium arises in continuity with the blood-vessels of the pellucid 
area, and is in no wise different from them; the myocardium, on 
the other hand, arises from the splanchnic mesoblast. The heart 
is thus to be regarded as a portion of the embryonic vascular 
system, specially provided with a muscular wall for the propul- 
sion of the blood. The first indication of the heart is a thicken- 
ing of the splanchnopleure of the amniocardiac vesicles, which 
forms the primordium of the myocardium. This is situated a 
short distance lateral to the hind-brain region of the embryo, and 
makes its appearance between the stage of 3 and 5 somites. 
The endocardium soon appears between the thickened ento- 
derm and the myocardium, in the form of a delicate endothelial 
vessel on each side, continuous with the extra-embryonic blood- 
vessels. This is, indeed, the place where the blood-vessels first 
