STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 831 



1 1. Development. 



We have already described in detail the first formation of 

 the alimentary tract so far as we have been able to work it out, 

 and we need only say here that the anterior and posterior ends 

 of the canal become first formed, and that these two parts 

 gradually elongate, so as to approach each other ; the growth of 

 the posterior part is, however, the most rapid. The junction of 

 the two parts takes place a very short distance behind the 

 opening of the bile duct into the intestine. 



For some time after the two parts of the alimentary tract 

 have nearly met, the ventral wall of the canal at this point is 

 not closed ; so that there is left a passage between the alimentary 

 canal and the yolk-sack, which forms a vitelline duct. 



After the yolk-sack has ceased to be visible as an external 

 appendage it still persists within the abdominal cavity. It has, 

 however, by this stage ceased to communicate with the gut, so 

 that the eventual absorption of the yolk is no doubt entirely 

 effected by the vitelline vessels. At these later stages of de- 

 velopment we have noticed that numerous yolk nuclei, like 

 those met with in Teleostei and Elasmobranchii 1 , are still to be 

 found in the yolk. 



It will be convenient to treat the history of sections of the 

 alimentary tract in front of and behind the vitelline duct 

 separately. The former gives rise to the pharyngeal region, the 

 oesophagus, the stomach, and the duodenum. 



The pharyngeal region, immediately after it has become 

 established, gives rise to a series of paired pouches. These may 

 be called the branchial pouches, and are placed between the 

 successive branchial arches. The first or hyomandibular pouch, 

 placed between the mandibular and hyoid arches, has rather 

 the character of a double layer of hypoblast than of a true 

 pouch, though in parts a slight space is developed between its 

 two walls. It is shewn in section in Plate 37, fig. 43 (h.m), from 

 an embryo of about 10 millims., shortly before hatching. It 



1 For a history of similar nuclei, vide Comp. Embryol., Vol. II., chapters III. 

 and IV. 



