<52 HEAKT AND EMBRYONIC BLOOD-VESSELS 



of the interval between the germ-layers (Ziegler). According to another account, the vessels 

 appear first as cords or strings of cells which arrange themselves round a lumen, and form the 

 endothelium of the vessel wall (Riickert). The vessels in terms of both these interpretations 

 are intercellular spaces ; but according to still other observers, the extension of the channels 

 is brought about by the formation of spaces within the vaso-formative cells, which are converted 

 into vessels by being linked up together. 



The vaso-formative cells are most generally regarded as mesodermic in origin ; but they 

 are considered by some as being derived independently from the entoderm, forming thus> an 

 ontodermic, as distinguished from the mesodermic mesenchyme. 



In Tarsius, it will be recollected, a ring-shaped thickening of the entoderm was mentioned as 

 giving rise to a band of middle-layer cells related to the formation of the vessels. According to 

 Hubrecht's account, therefore, the vascular mesenchyme arises direct from the entoderm. It 

 may be noted that the disposition of the ring closely corresponds with the primitive vessels 

 of the early human embryo as described by Eternod. 



In the absence of detail for the early phases of the primate heart, the stages in 

 the rabbit- embryo may be taken as a type for the development of the parts. 



II 



I & 



FIG. 88. TBANSVEESE SECTION THBOUGH THE REGION 

 OF THE HEART IN A BABBIT EMBBYO OF NINE DAYS, 



SHOWING THE COMMENCING FUSION OF THE TWO 



TUBES. Y- (Kolliker.) 



jt 3-> J u 8 u l ar veins ; ao, aortee ; ph, pharynx ; sow, 

 somatopleure of body-wall ; bl, bilaminar portion of blasto- 

 derm forming pro-amnion ; ect, ent, its two layers (ecto- 

 derm and entoderm) ; p, pericardium ; spl, splanchnopleure ; 

 afy, outer wall of heart ; ih, endothelial lining of heart ; 

 e', septum between the tAvo heart-tubes. 



FIG. 89. EMBBYO BABBIT OF EIGHT 



DAYS AND EIGHTEEN HOUBS, WITH 



NINE PBOTOVEBTEBBJE ; VENTRAL 



ASPECT. V- (Kolliker.) 



The heart is still a double tube. 



At an early period, before the splanchnopleuric folds have begun to fold in to 

 form the fore-gut, it will be seen (fig. 86) that the pericardial portion of the 

 ccelom is occupied by a fold of the splanchnic mesoderm. This fold becomes 

 subsequently closed-in to form the muscular wall of the heart. It encloses a 

 second tube composed of flattened cells, which becomes the endothelial lining 

 of the heart. Authorities differ as to the origin of these cells, some deriving them 

 from the mesoderm, while others trace them direct from the entoderm, either in 

 the form of an evagination or as a solid cord of cells. When the splanchnopleuric 

 folds bend in to form the floor of the fore-gut, the two tubes are brought together 

 (fig. 88) below the pharynx. They at first lie side by side, but soon fuse into 

 a single median tube by the absorption of the dividing septum. The heart- 

 tube remains attached to the gut by a mesentery, the mesoeardium poster ius 

 (fig. 90). 



