STUDIES ON THE DEVELOPMENT OF THE HORSE. 303 



are connected by protoplasmic bands with the vessels of the unsplit mesoderm 

 (% 28). 



Except within the trophoblastic discs, the endoderm beyond the margin of the 

 unsplit mesoderm also consists of a single layer of polygonal cells (fig. 30), but 

 opposite the discs the endoderm cells proliferate to form tubercles which project 

 into the cavity of the yolk-sac (figs. 29 and 32). 



The first indication of a trophoblastic disc is a local thickening of the coagulum 

 lying between the trophoblast and endoderm, presumably the result of the increased 

 activity of a group of trophoblastic cells (fig. 30). The first indication of a yolk- 

 sac tubercle is an ingrowth from the thickened coagulum which causes the endoderm 

 to bulge into the cavity of the yolk-sac (figs. 31 and 31a). The endoderm cells in 

 contact with the globular ingrowth from the coagulum stain more deeply and are 

 decidedly more granular than the surrounding cells (fig. 31a). It may hence be 

 inferred that the coagulum is attacked by the endoderm cells in its immediate 

 vicinity. As the coagulum expands and projects further into the cavity of the 

 yolk-sac it acquires an almost complete investment of endoderm cells with relatively 

 large nuclei. A section through the centre of a growing tubercle (still connected 

 by a short stalk with the subtrophoblastic layer of coagulum) is represented in 

 fig. 32, while fig. 32a represents a section through the edge of the same tubercle. 



If, as is usually the case, the coagulum forming the kernel of the tubercle reaches 

 a considerable size, the capsule of endodermic cells ruptures, with the result that the 

 coagulum projects freely into the cavity of the yolk-sac (fig. 29). Just as it is im- 

 possible to say to what extent the uterine milk is modified as it passes through the 

 trophoblastic discs to form the coagulum, it is impossible to say how the contents of 

 the tubercles are modified by the endodermic cells forming their capsules. 



5. The Mesoderm. 



At the middle of the third week the mesoderm is in the act of splitting in the 

 region of the mesodermic somites ; at the end of the third week, as already mentioned, 

 the splitting of the mesoderm into somatic and splanchnic layers has extended some 

 distance beyond the embryo (fig. 34 and text-fig. 12). The inner portion of the space 

 resulting from the splitting becomes the coelom, the outer forms the exoccelom (text- 

 fig. 12). As fig. 34 and text-fig. 12 indicate, the greater part of the mesoderm at the 

 end of the third week is still unsplit, and lies between the endoderm and trophoblast. 

 This unsplit mesoderm is in contact neither with the trophoblast nor the endoderm ; 

 it occupies a space between these layers, and supports the vitelline vessels carrying 

 blood to and from the embryo. The blood comes direct from the two aortse by the two 

 vitelline arteries (text-fig. 7). The left artery (l.v.), very much larger than the right 

 (figs. 27 and 35), eventually bifurcates and encircles the blastocyst as the sinus 

 terminalis (fig. 34) ; the left artery, before bifurcating to form the sinus, gives off 

 numerous branches, some of which anastomose with branches from the small right 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 7). 44 



