in THE ALIMENTAEY CANAL 193 



temporary absence of luinen so frequently found in the development 

 of eventually hollow m^ans from a richly yolk-laden rudiment. 

 An idea of how it has come about will be got from an inspection of 

 the various stages of the development of the alimentary canal of 

 Polypterus as shown in Fig. 80 on p. 146. During early stages the 

 archenteric cavity is seen to be widely patent throughout, except 

 that there is no mouth opening. During the later stages of develop- 

 ment, inn i KM I lately prior to the canal becoming functional, its walls 

 throughout the region between the fore-gut and the cloaca become 

 closely apposed. so as almost entirely to obliterate the cavity. Later 

 on the walls recede from one another and the lumen becomes again 

 patent. 



It would obviously be merely a slight accentuation of this 

 modification of development for the cavity to be completely obliter- 

 ated for a time. A still further modification would be brought 

 about by the omission altogether of the original hollow stage from 

 the ontogenetic record. This actually occurs in the case of the 

 fore-gut in those Vertebrates in which this region of the enteric 

 rudiment is yolk -laden : where, on the other hand, the yolk is 

 practically completely concentrated in the mid-gut region as in 

 meroblastic Vertebrates it does not occur as a rule. 



The most striking temporary occlusions of the alimentary canal 

 during development have to do with its terminal apertures. Thus 

 there is not a single existing Vertebrate, so far as is known, in 

 which the mouth opening persists from the gastrular stage, or in 

 which even any connexion has so far been traced between the 

 definitive mouth opening and the protostoma. In every case, even 

 in Amphioxus, the mouth opening develops comparatively late as a 

 secondary perforation. This modification of development is in the 

 present writer's opinion to be attributed to the entire dependence of 

 members of the Vertebrate phylum upon food-yolk during early 

 stages of their development, the need for a functional mouth having 

 thus disappeared. 



The auteroposterior extent of this occlusion of the alimentary 

 canal in the region of the oral opening differs in different sub- 

 divisions of the phylum. It may include a large part of the 

 stomodaeal as well as the endodermal portion of the buccal cavity as 

 in the Lung-fishes (p. 148) but more usually it is confined to the 

 boundary between the two, i.e. to the site of the original mouth 

 opening the closely apposed ectoderm and endoderm being at this 

 level continuous across the site of the future opening as the velar 

 mi'inbrane (p. 145). The secondary perforation by which the 

 alimentary canal comes to communicate with the exterior at its 

 front end is in the case of some larval Vertebrates (e.g. Lepi- 

 dosiren} closely correlated with the commencement of pharyngeal 

 respiration but where the development is embryonic it commonly 

 still takes place long before the existence of any obvious functional 

 need (e.g. Chick, fourth day). At its hinder end the archenteron is, 



VOL. II 



