STUDIES ON THE DEVELOPMENT OF THE HORSE. 309 



3. The Alimentary Canal. 



At the end of the third week the alimentary canal is closed in front and behind, 

 but the middle portion is in free communication with the yolk-sac (fig. 33). An 

 ectodermic involution in front of the fore-gut represents the mouth, while an endo- 

 dermic outgrowth from the hind-gut represents the allantois (fig. 33). The mouth 

 (stomodseum) is an irregular deep pit between the naso-frontal process and the first 

 (mandibular) pair of branchial arches (fig. 8). The floor of the pit which separates 

 the embryonic mouth from the pharynx consists of only two layers (fig. 33), a layer of 



Text-fig. 6. — Diagram of branchial pouches and branchial arches, l.b.p. to IV. b. p., the four 

 branchial pouches ; I. b. a. to IV. 6. a., the four branchial arches ; l.b.g. and III. b.g., the first 

 and third branchial grooves ; l.a.a., first aortic arch ; II. a.a., second aortic arch ; mo., mouth 

 pit ; I. , possible lung rudiment ; t. , tuberculum impar. 



ectoderm and a layer of endoderm, the latter intimately related to the notochord and 

 the mandibular arches. Immediately behind this partition (the bucco-pharyngeal 

 membrane) between the mouth and the pharynx a diverticulum projects upwards 

 towards the notochord (fig. 37), which Professor Robinson regards as Seessel's pouch. 

 The Fore-gut. — At the end of the third week the pharyngeal part of the fore-gut 

 is especially interesting because of the presence of branchial or gill pouches and their 

 related branchial arches. In the chick there are during development four pairs of 

 pouches and five pairs of arches, and for a time each of the three foremost pouches 

 opens to the exterior and thus gives rise to imperfect gill clefts. In man at the end 

 of the third week there are four pairs of branchial arches and four pairs of branchial 



