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 Plolliker and others. The second is sup- 

 ported by Riickert, 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 and continuous. But it is obvious that such observations 

 do not in themselves demonstrate the existence of an independent 

 ingrowing primordium; they are not altogether inconsistent with 

 the view that the blood-vessels differentiate from cells in situ. 

 Within the embryo itself parts of certain vessels appear in sections 

 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. But such appear- 

 ances seen in sections may be deceptive, as Evans has shown by 

 injections of the ingrowing vascular system of early chick embryos. 

 The entire system appears in such injections to be continuous 

 from the first and there was found no evidence of independently 

 formed parts. 



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 



