RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 345 



umbilicus, where the cavity of the pericardium is separated from the extra-embryonic 

 coelom by the mesodermal septum transversum. 



At its dorsal boundary on each side, at the level of the fourth branchial pouch 

 and the cranial border of the duct of Cuvier, the pericardium communicates with a 

 pleuro-pericardial canal (fig. 58, PI. XIX). 



The Pleuro-'pericardial Canals. — Each pleuro-pericardial canal commences from 

 the dorsal part of the pericardial cavity, at the level of the cranial border of the 

 duct of Cuvier. It runs from the more cranially situated to the more caudally 

 situated end of the bend of the embryo, terminating, in the latter situation, in 

 what will eventually be the peritoneal portion of the coelom. As it passes between 

 its two terminations it lies medial to the duct of Cuvier ; dorsal to the sinus venosus, 

 by which its ventral boundary is invaginated ; and ventral to the dorsal aorta 

 and the posterior cardinal vein of the same side (text-figs. 25, 26, pp. 341, 342, and 

 fig. 58, PL XIX). 



DESCRIPTION OF FIGURES ON PLATE XIX. 



Fig. 54. Dorsal view of the caudal portion of a reconstruction of a horse embryo 11 mm. long. 



Fig. 55. Dorsal view of the cranial portion of the reconstruction, and ventral view of the most caudal 

 part of the caudal portion, of a horse embryo 11 mm. long. Both portions have been partially dissected. 



Fig. 56. Left lateral view of a reconstruction of a horse embryo 11 mm. long. A portion of the heart 

 is shown, and the lines of incision for the removal of the ventral wall of the pharynx and the dorso-lateral 

 wall of the left pleuro-pericardial canal are shown. 



Fig. 57. View of a median section of a reconstruction of a horse embryo 11 mm. long. The heart has 

 not been divided, and its left side is shown. 



Fig. 58. Right lateral view of a reconstruction of a horse embryo 11 mm. long. The reconstruction 

 has been dissected to show the relations of the branchial pouches and the otic vesicle to the blood-vessels 

 and the heart. 



Fig. 59. Dorsal view of the caudal end of a reconstruction of a horse embryo 11 mm. long. Dissected 

 to show terminal part of right dorsal aorta. 



Fig. 60. Ventral view of the caudal end of a reconstruction of a horse embryo 11 mm. long, showing 

 the blood-vessels. 



Fig. 61. Dissection of the ventral portion of the caudal end of a reconstruction of a horse embryo 

 11 mm. long, showing cavities of allantoic diverticulum and hind-gut. 



Fig. 62. Ventral view of a reconstruction of the heart of a horse embryo 11 mm. long. 



Fig. 63. View of the dorsal aspect of the ventral wall of the pharynx of a horse embryo II mm. long, 

 showing section of the first three branchial arches and sections of the first and second aortic arches. 



Note. — Figs, referred to, other than those on Plate XIX, are in the Plates of Professor Cossar Ewart's 

 paper. 



[Key to Lettering 



