148 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



Later on each ridge becomes sharply bent, about the middle of its 

 length, in such a way as to give the buccal opening a rhomboidal 

 shape and at the same time to mark off the ridge into a maxillary 

 portion in front and a mandibular portion behind. In Anura a ^ 

 somewhat similar arrangement is found. 



"Endodermal" Section of Buccal Cavity. — The fully 

 developed buccal cavity has incorporated in it a posterior portion — 

 varying in relative extent in different Vertebrates — which is de- 

 rived not from the ectoderm but from the anterior portion of the 

 " endodermal " enteric rudiment. The simplest way in which this 

 portion becomes added to the anterior portion is seen in those 

 Vertebrates in which the anterior part of the enteric cavity is patent 



throughout develop- 

 ment. In this case 

 the velar membrane 

 simply ruptures — its 

 remnants soon be- 

 coming absorbed — 

 and the stomodaeal 

 cavity is thrown into 

 open communication 

 with the enteric 

 cavity. This is the 

 case in certain 

 Anura (Sana) and 

 in Amniota. 



In many Verte- 

 brates no velar mem- 

 brane is present, owing to the fact that the foregut either becomes 

 solid for a time (Polypterus, Fig. 80, D-G) or is so at the beginning 

 (Teleostei, Urodela, Lepidosiren and Protopterus). In such cases the 

 peripheral layer of the yolky foregut rudiment gradually assumes 

 an epithelial character and the yolk along its middle breaks down, 

 so that a cavity arises — continuous with the stomodaeal cavity and 

 forming the hinder section of the definitive buccal cavity. The pro- 

 portion which this posterior portion bears to the anterior section 

 derived directly from the outer surface is very different in different 

 groups. It apparently attains its maximum in Teleosts where 

 it forms practically the whole of the buccal cavity. 



Points of critical importance to the germ-layer theory are raised 

 in this connexion by the fact that teeth, organs belonging originally to 

 the outer surface, are developed in this posterior region of the buccal 

 cavity from yolky " endoderm." This is well seen in a Urodele, or 

 a lung-fish such as Lepidosiren or Protopterus (Fig. 82). The attempt 

 is made to get round this difficulty by assuming that the layer of 

 epithelium which makes its appearance over the surface of the buccal 

 rudiment, and in relation with which the teeth develop, is really 

 an ingrowth from the ectoderm. 



Pig. 81. 



-Ventral view of head region of embryos of Scyllium 

 canicula. (After Sedgwick, 1892.) 



A, 7-8 mm. 



; B, slightly more advanced than A ; C, 11-12 mm. 

 D, 16 mm. 



