310 PROFESSOR J. COSSAR EWART. 



pouches, but only the first two pairs of pouches and the first two pairs of arches (the 

 mandibular and hyoid) are well developed, and even the large first pouch has never 

 been found opening to the exterior. 



In the 21-days horse the branchial pouches and arches generally agree with the 

 corresponding structures in man. The first pouch (figs. 9 and 33 and text-fig. 6) 

 projects outwards and upwards between the first (mandibular) and second (hyoid) 

 a relies until it is only separated from the first branchial groove (fig. 8 and text-fig. 6) 

 by a thin partition (consisting of endoderm and ectoderm) which eventually takes 

 part in forming the tympanic membrane. The second branchial pouch (fig. 9 and 

 text-fig. 6), though smaller than the first, extends outwards and then upwards 

 towards the otic sac (fig. 38). thus filling up the greater part of the space between 

 the second (hyoid) and third branchial arches. On the way it approaches the second 

 branchial groove (text-fig. 6) ; but this groove is neither as long nor as deep nor 

 yet as intimately related to its pouch as the groove between the mandibular and 

 hyoid arches. 



The third branchial pouch (figs. 9 and 39), decidedly shorter and narrower than 

 the second, extends outwards and slightly upwards towards the space between the 

 third and fourth branchial arches (text-fig. 6) to end some distance from the shallow 

 third indication of a branchial groove (fig. 8). In the horse, as in man, there is only 

 a minute diverticulum (text-fig. 6) representing the fourth branchial pouch of the 

 chick, and only at the most a faint indication of a branchial groove lying behind the 

 rudimentary fourth branchial arch (text-fig. 6). 



In the floor of the pharynx of man at the end of the third week there is a small 

 mesial tubercle (tuberculum impar) between the mandibular and hyoid arches, a 

 forked elevation (furcula) between the second and third arches, and furrows lead- 

 ing into the branchial pouches ; the tubercle is the first indication of a tongue, 

 while the furcula gives rise to the epiglottis. In the 21-days horse there is a 

 small mesial prominence in the floor of the pharynx between the first and second 

 branchial arches (text-fig. 6), and furrows leading to the branchial pouches, but 

 no rudiment of either larynx or epiglottis. It may be mentioned that the fore- 

 gut expands behind the fourth branchial pouch and gives off at each side a 

 shallow diverticulum (text-fig. 6). These diverticula may represent rudiments of 

 the respiratory system. 



The Mid-gut and the Hind-gut.— -The mid-gut, together with the anterior part 

 of the hind-gut, communicates with the yolk-sac by means of a broad yolk-stalk (fig. 33 

 and text-fig. 12). In the region of the yolk-stalk the cavity of the hind-gut has the 

 form of a narrow channel (fig. 50), but behind the yolk-stalk it assumes the form of 

 a wide flattened tube (fig. 51), the end of which projects upwards into the mass of 

 cells occupying the space between the notochord and the terminal portion of the 

 spinal cord : this dorsal diverticulum (figs. 33 and 52) seems to represent the cloacal 

 chamber. From the ventral aspect of the hind-gut the allantoic diverticulum (fig. 33) 



