HEART AND EMBRYONIC BLOOD-VESSELS 



63 



ect 



In the earliest stage described for the human embryo (thirteenth day) the 

 heart-tubes are still separate from one another. We owe to Eternod ' a description, 

 arrived at by reconstruction from sections, of the vessels of an embryo of the 

 thirteenth day, when the channels are still in the course of formation in the mesen- 

 chyme. The heart-tubes are simply dilated portions of a continuous sinus-like 

 vessel which surrounds the mouth of the yolk-sac and ends in a common stem in 

 the abdominal stalk, which is in turn distributed to the chorion (fig. 91). The two 

 tubes are united for a short distance under the fore-gut into a single vessel, which is 

 the rudiment of the aortic bulb. From this two vessels sweep back on each side 

 of the notochord, which is still in the stage of the notochordal plate : these are the 

 primitive aortas, and the loops between the ventral bulb and the dorsal aorta are 

 the first or primitive aortic arches. 2 Behind, the aortae, sweeping past the neurenteric 

 canal, bend round the caudal 

 end of the embryonic axis 

 into the abdominal stalk, and 

 pass in this to the chorion. 

 Where the abdominal stalk 

 becomes continuous with the 

 yolk-sac the sinus-like vessel 

 is joined by a vascular loop 

 from the back and under side 

 of the yolk-sac (sinus ensi- 

 forme), but the vitelline veins 

 proper, which afterwards con- 

 vey the blood from the vitel- 

 line circulation to the heart, 

 have not yet been laid down. 



It would thus seem that 

 there is a circulation set up 

 between embryo and chorion 

 at a very early stage, before 

 even the yolk circulation is 

 established. This is to be 

 correlated with the vestigial 

 condition of the yolk-sac, and 

 is another instance of the 



remarkable series of variations from the ordinary type which the development of 

 the primate embryo exhibits. 



In the next stage of which we have complete detail, a human embryo of 

 fifteen days (His' embryo Lg., fig. 92), and for the lower Primates an embryo of 

 Cercocebus cynomolgus described by Selenka, the yolk circulation is fully estab- 

 lished, and the course of the vessels has become so modified as to conform to 

 the condition described for the lower mammals with a large yolk-sac and a vascular 

 area. 



The heart is now a single tube, and shows a subdivision into an auricular, 

 a ventricular, and a bulbar part. It receives three veins on each side, which join 

 a transverse vessel placed in the septum transversum, named the sinus venosus. 



The three pairs of veins are the vitelline, running in the splanchnopleure from 

 the yolk-sac ; the allantoic, running in the edges of the somatopleure and 



1 Anat. Anzeiger, xv. 1899. 



2 It may be mentioned that even at this very early stage, according to Eternod's descriptions, there 

 are indications of the rudiments of two, perhaps three, connecting vessels on each side representing 

 future aortic arches. 



FIG. 90. SECTION THROUGH THE REGION OF THE HEART IN 

 A RABBIT EMBRYO OF TEN DAYS, AFTER THE TWO TUBES 



HAVE UNITED INTO A SINGLE MEDIAN ORGAN. (Kb'lliker.) 



ao, descending aortee ; ba, bulbus aortas ; ah, its external 

 wall ; i/c, its endothelial lining ; mp, mesocardium posterius, 

 uniting the heart to the ventral wall of the pharynx, ph, and 

 here separating the pleuropericardial ccelom, p, into two 

 halves, which are, however, united on the ventral side of the 

 heart ; ent, entoderm of yolk-sac ; df, its mesoderm ; df, meso- 

 derm of pharynx ; h, mesoderm of somatopleure ; ect, ectoderm. 



