RECONSTRUCTION MODEL OF A HORSE EMBRYO TWENTY-ONE DAYS OLD. 337 



they are separated from one another by the second aortic arch. Each is provided 

 with a dorsal and a ventral diverticulum. 



The ventral ends of the first pair of branchial pouches are continuous with trans- 

 verse grooves in the ventral wall of the pharynx, which are separated from one 

 another by the median longitudinal ridge which unites the tuberculum impar with 

 the transverse ridge between the second arches (fig. 63, PI. XIX). 



The ventral ends of the second branchial pouches are connected together by a 

 transverse sulcus which crosses the ventral wall of the pharynx immediately caudal 

 to the second arches (fig. 63, PI. XIX). 



The third branchial pouch is a shallow cranio-caudal evagination of the corre- 

 sponding ventro-lateral border of the triangular pharyngeal cavity, and the fourth 

 pouch is of similar character but of much smaller size (fig. 57, PI. XIX). 



The lateral borders of the third and fourth pouches can be seen in fig. 58, PI. XIX, 

 lying between the dorsal aorta and the dorsal wall of the right atrium immediately 

 cranial to the duct of Cuvier. 



Caudal to the region of the fourth branchial pouches the gut rapidly expands, 

 and immediately dorsal to the cranial end of the sinus venosus its ventral wall dips 

 slightly in the median plane, whilst, at the same level, the ventral part of each 

 lateral wall expands into a shallow pouch. It is probable that the median depression 

 and the lateral diverticulse represent the rudiments of the respiratory system. This 

 supposition is supported by the fact that each lateral diverticulum abuts against the 

 ventro-medial wall of the corresponding pleuro-pericardial canal (text-fig. 26, p. 342, 

 and fig. 57, PL XIX). 



The cavities of the first and second branchial pouches are separated from the 

 corresponding branchial cleft by ectoderm and entoderm only, but between the 

 cavities of the third and fourth pouches and the exterior there is mesoderm as 

 well as ectoderm and entoderm. 



The Branchial Clefts. 



Three branchial clefts are visible on the surface (fig. 56, PI. XIX). They are 

 vertical clefts, broadest and deepest at their ventral ends, which lie quite close to the 

 pericardium, but the depth of the clefts is considerably less than that of the corre- 

 sponding pouches (fig. 63, PI. XIX). 



The Branchial Arches. 

 Four arches are visible on the exterior aspect of the pharyngeal region, but only 

 three on the inner aspect. On the outer aspect the first three form vertical ridges, 

 of which the first is the longest and most massive, whilst the fourth is a rounded 

 eminence (fig. 56, PL XIX). The ventral ends of the first arches are joined together 

 in front of the cranial border of the pericardium. The ventral ends of the three 

 posterior arches abut against the dorsal border of the bulging pericardial region. 



