STUDIES ON THE DEVELOPMENT OF THE HORSE. 311 



extends backwards and upwards into the thick layer of mesoblast surrounding the 

 caudal end of the embryo. 



4. The Notochord. 



From Martin's statements it may be assumed that the notochord in the horse 

 agrees in its origin with that of the mole. As in the mole, the endoderm cells under- 

 lying the medullary canal in front of the primitive streak are enlarged and arranged 

 first to form a groove and then a cord (the chorda dorsalis), the thickened posterior 

 end of which may contain a canal continuous with the neurenteric canal. At the 

 middle of the third week large cells, continuous with ectodermic and endodermic 

 cells at the anterior end of the primitive streak, form, a groove immediately under 

 the medullary groove. At the end of the third week the place of this groove is 

 occupied by the notochord. In the last fifteen sections through the tail end of the 

 embryo there is no indication of a notochord, but in the sixteenth section (fig. 52) 

 the notochord, nearly one-third the size of the spinal cord, is seen to the right of the 

 middle line : like the spinal cord, it has a distinct central canal. '* Though eccentric 

 in position near its origin, the notochord soon occupies the middle line and lies 

 immediately below the spinal cord. If the drawings of transverse sections through 

 the trunk (figs. 43 to 52) are referred to, it will be observed that the notochord and 

 its canal vary both in shape and size. The sections through the head and pharynx 

 (figs. 37 to 40) show that the notochord varies in shape and size and is inti- 

 mately connected with the endoderm of the anterior third of the roof of the pharynx 

 (figs. 33 and 39) and with the hind-brain. 



5. Heart and Blood-vessels. 



The embryo when first exposed was seen to be surrounded by a countless number 

 of blood-vessels engaged in carrying blood to and from the yolk-sac (figs. 5, 34 and 35). 

 Of these vessels the most obvious were the left vitelline artery, the sinus terminalis, 

 and the large vitelline veins proceeding to the large bulging heart occupying the 

 space between the pharynx and the middle portion of the trunk (fig. 33). The heart 

 in the horse at the end of the third week, like the heart in a 3-day s chick, consists 

 of a sinus venosus, an atrium, a ventricle, and a truncus arteriosus."!" In a 3 -days 

 chick, as in man about the end of the third week, there are five pairs of aortic arches, 

 but in the 3-weeks horse only the first two pairs of arches (text-fig. 7) have made 

 their appearance. There were neither rudiments of a heart nor yet of blood-vessels 

 in Martin's embryo ; hence it may be assumed that in the horse during the second 

 half of the third week there is (l) the vascularisation of the yolk-sac and the 

 formation of the vitelline veins ; (2) the fusion of these veins to form a single tubular 



* The notochord al canal, unlike the neural canal, is closed, does not open into the amniotic cavity, 

 f Professor Robinson in his description of the model recognises (1) a sinus venosus, (2) a sinu-atrial canal, 

 (3) a ventricle, (4) an atrio-ventricular canal, (5) a bulbus cordis, and (6) a truncus aorticus. 



TRANS. ROY. SOC. EDIN. VOL. LI, PART II (NO. 7). 45 



