STUDIES ON THE DEVELOPMENT OF THE HORSE. 325 



represented by endoderm (text-fig. 12). Beyond the sinus the yolk-sac endoderm is 

 intimately connected with the trophoblast (fig. 34). Except opposite the discs the 

 yolk-sac endoderm consists of a single layer of cells ; opposite the discs, where it takes 

 part in forming the yolk-sac tubercles, the endoderm may consist of two layers of 

 cells (fig. 32). 



At the middle of the third week the splitting of the mesoderm is only beginning, 

 and the amnion is only represented by indistinct lateral ridges (text-fig. 11). At the 

 end of the third week the amnion is complete and the mesoderm is split so as to 

 form a coelom and a small exocoelom (text-fig. 12). The outer (somatic) layer of 

 mesoderm together with a layer of ectoderm forms the amnion ; further, by uniting 

 with the trophoblast the somatic mesoderm gives rise to the chorion of the embryonic 

 area (text-fig. 12). The inner (splanchnic) layer of mesoderm by uniting with 

 endoderm forms the yolk-stalk and the upper free and only complete part of the 

 yolk-sac. 



The unsplit mesoderm (text-fig. 12) which extends from the exocoelom to the 

 sinus terminalis is highly vascular ; it receives blood from the embryo by the vitelline 

 arteries and returns it to the embryo by the vitelline veins (fig. 34). 



There is no indication of an allantoic diverticulum at the middle of the third 

 week, but at the end of the third week there is a considerable mass of vascularised 

 allantoic mesoderm at the caudal end of the embryo, into which extends from the 

 hind-gut a small allantoic diverticulum (fig. 33 and text-fig. 17). 



Martin's embryo, the age of which is almost certainly 17 or 18 days, has four pairs 

 of mesodermic somites (fig. 7). Hausmann's 19-days embryo (text-fig. 19) has 

 probably ten somites, but the 21-days embryo has over twenty somites (figs. 10 and 

 11). Martin's embryo in shape resembles the sole of a shoe; my 21-days embryo 

 is hook-shaped (figs. 7 and 8). That this strongly bent embryo is older than Martin's 

 so-called 21-days embryo is indicated by its size, by the phase reached in the develop- 

 ment of the nervous, alimentary, and circulatory systems, and especially by the 

 complete amnion and the allantoic diverticulum. The nervous system is represented 

 by a fore-brain with optic vesicles, by an indistinct mid-brain, by a hind-brain adjacent 

 to which are otic vesicles, and by a spinal cord completely closed except at the caudal 

 end, where it opens by a neuropore into the cavity of the amnion (fig. 33). 



The alimentary system consists (1) of a fore-gut from which branchial pouches 

 project outwards towards shallow branchial furrows (text-fig. 6) lying between 

 branchial arches, (2) of a mid-gut which communicates by a yolk-stalk with a large 

 yolk-sac (fig. 34), and (3) of a short hind-gut from which a cloacal rudiment projects 

 upwards and an allantoic diverticulum extends backwards (fig. 33 and text-fig. 17). 



The circulatory system is represented by a sinus venosus, an atrium, a ven- 

 tricle, and a bulbus arteriosus ; by two pairs of aortic arches, two dorsal aortae, 

 and by vitelline and other arteries, vitelline, cardinal, umbilical, and other veins 

 (text-fig. 7). 



