STUDIES ON THE DEVELOPMENT OF THE HORSE. 329 



Plate XV.* 



Fig. 36. Section through fronto-nasal process of the 21-days embryo, am., amnion; am.c, amniotic 

 cavity; f.b., cavity of fore-brain; p.o.v., primary optic vesicle. 



Fig. 37. Section through head end of notochord (nc). m.b., mid-brain ; f.g. fore-gut ; s.p., Seessel's 

 pouch ; sto., stomodseum ; 1 a.a., first aortic arch. 



Fig. 38. Section through otic vesicles (o.v.). h.b., hind-brain ; d.a., dorsal aorta ; f.g., fore-gut, 

 from which notochord projects upwards towards floor of hind-brain ; 2 b.p., second branchial pouch ; 

 d.a., dorsal aortse formed by union of first two aortic arches ; jit., atrium ; b.co., bulbus cordis ; 

 pc, pericardium. 



Fig. 39. Section through third branchial pouch (3 b.p.). at., atrium ; a.v.c, atrio-veutricular constric- 

 tion ; b.co., bulbus cordis; d.a., dorsal aorta; h.b., hind-brain; pc, pericardium; v., ventricle. 



Fig. 40. Section through fourth branchial pouch (4 b.p.). at., atrium; at.v., atrio-ventricular valve; 

 b.co., bulbus cordis; d.a., dorsal aorta; h.b., hind-brain; v., ventricle. 



Plate XVI. 



Fig. 41. Section through anterior part of sinus venosus (s.v.). am., amnion; d.a., dorsal aorta; 

 f.g., fore-gut; nc, notochord; pp.c, pleuro-pericardial cavity; s.c, spinal cord; v.v., vitelline vein. 



Fig. 41a. Section through middle part of trunk in a line with above, m.g., mid-gut ; m.p., muscle plate; 

 p.d., pronephric duct; u.v., umbilical vein; w.b., Wolffian body. Other abbreviations as before. 



Fig. 42. Section through trunk and middle portion of sinus venosus (s.v.). d.c, duct of Cuvier ; 

 lu., lung; v.v., vitelline vein; c.v., branch of vitelline vein. Other abbreviations as before. 



Plate XVII. 



Fig. 43. Section through posterior part of sinus venosus (s.v.) and the terminal portions of vitelline 

 veins (v.v.). pc, pericardium. Other abbreviations as before. 



Fig. 44. Section through middle part of trunk in region of yolk-stalk, sp., splanchnopleure. Other 

 abbreviations as before. 



Figs. 45 and 46. Sections through posterior third of trunk. Note that the amnion (am.) is folded to 

 form longitudinal ridges, c.v., posterior cardinal vein. Other abbreviations as before. 



Fig. 47. Section near the point where the dorsal aorta divides in two. Abbreviations as before. 



Plate XVIII. 



Figs. 48 and 49. Sections nearer caudal end than fig. 47, showing two aortse (d.a.). Abbreviations as 



before. 



Fig. 50. Section through hind-gut (h.g.) immediately in front of posterior wall of yolk-stalk. Note the 

 triangular form of the spinal cord (s.c), the mesodermic somites (m.p.), cardinal veins (c.v.), and the two 

 aortse (d.a.). 



Fig. 51. Section through the posterior wall of the yolk-stalk (y.st.). al.b., allantoic branches of aorta ; 

 h.g., hind-gut. Other abbreviations as before. 



Fig. 52. Section through cloaca (cl.) immediately in front of opening of spinal cord into amniotic cavity. 

 The notochord nc, is tubular, and lies at one side of the middle line. Note the wide allantoic diverticulum 

 (al.d.) and the crescent-shaped amniotic cavity (am.c). Other abbreviations as before. 



Fig. 53. The tenth section from the caudal end of the embryo, am., amnion; am.c, amniotic 

 cavity; al.d., terminal portion of allantoic diverticulum; en., caudal neuropore ; s.c, spinal cord; sp., 

 splanchnopleure ; u.v. umbilical vein. 



* The figures in Plates XV-XVIII are all magnified 75 diameters. 



