HF.AKT AND EMBRYONIC BLOOD-VESSELS 



61 



Early stages in the development of heart and embryonic vessels. 



The first embryonic blood-vessels are laid down in the splanchnopleure, and the 

 earliest channels to appear are two short tubes on each side of the head end 

 of the embryonic axis. These form the double rudiment of the heart. 



not.pl. 



ent 



FIG. 86. A. TRANSVERSE SECTION THROUGH THE HEAD OF AN EMBRYO RABBIT OF EIGHT DAYS 



AND FOURTEEN HOURS, WITH A PART OF THE PERIPHERAL BLASTODERM. * T 8 ' (Kolliker.) 



7?, h, rudiments of the heart ; sr, pharyngeal groove, with notochord-plate. 



B. PART OF THE SAME MORE HIGHLY MAGNIFIED. J^. (Kolliker.) 



n.p., neural plate (of hind-brain) ; n.g., neural groove ; n.f., neural fold ; ect, ectoderm ; ent, entoderm ; 

 mes, mesoderm ; p, paraxial mesoderm ; som, somatopleure ; spl, splanchnopleure ; per, pericardia! 

 coelom ; ah, fold of splanchnopleure which will form wall of heart ; ih, endothelial tube of heart ; 

 sio, wall of fore-gut; not.pl., notochord-plate. 



Various views are held as to the origin of the vessels in the embryo. One is that 'they grow 

 into the embryo from the vascular area, by a budding of the endothelial walls of the vessels 

 first laid down there (His) ; another, that the whole closed vascular system is produced by exten- 

 sion of two primary endothelial sacs, the heart-tubes (Rabl) ; a third, that they arise in sitv. 

 The third theory is on the whole most consonant with the appearances seen in mammalian 



ect 



\ 

 not.pl. 



cut 



spl pei 



FIG. 87. SECTION FROM THE SAME EMBRYO FARTHER FORWARD THAN THAT SHOWN 



IN THE PRECEDING FIGURE. (Kolliker.) 



Lettering as in fig. 86. 



embryos, but varying accounts are given of the actual mode of origin of the channels and the 

 source of the vaso-formative cells. According to one interpretation, the channels are spaces 

 in the mesenchyme which become converted into vessels by the transformation of the surrounding 

 cells into an endothelium ; and these spaces have been looked on hypothetically as the remains 



