gh. in THE ALIMENTAEY CANAL 145 



becomes extended into the form of a thick-walled tube — the rudiment 

 of the intestine (ent). From the stage of Fig. 80, E, onwards active 

 growth of the true tail or postanal region is taking place, and it is 

 noteworthy that, during this process, the endoderm retains for a 

 considerable time its continuity with the mass of actively growing 

 undifferentiated tissue at the tip of the tail and becomes drawn out 

 into a cylindrical postanal gut (pa.g). This remains conspicuous 

 for a time but eventually disintegrates and disappears completely. 

 The main mass of yolk -cells, forming the ventral wall of the 

 middle part of the enteron, gradually shrinks in volume, as the 

 yolk is absorbed and carried off by the circulating blood for distribu- 

 tion to the growing and developing tissues of the body, and eventually 

 the gut wall is no thicker in this region than it is elsewhere. 



Buccal Cavity. — The alimentary canal of the adult Vertebrate 

 commences with the buccal cavity which is in part at least — as 

 shown by the presence within it of placoid and glandular elements 

 corresponding with those of the skin — stomodaeal in its nature. The 

 stomodaeum however is not as a rule developed, as is so usually the 

 case in the Invertebrates, from a simple involution of the ectoderm 

 forming a depression of the surface below the general level. It 

 arises rather by the walling in of- an area on the ventral side of the 

 head through the development of ridge-shaped outgrowths. These 

 ridges may be termed respectively the maxillary ridge and the 

 mandibular ridge accordingly as they give rise later on to the upper 

 or to the lower jaw. 



The roof of the buccal cavity, or at least its anterior portion, is 

 simply to be looked on as part of the primitive ventral surface 

 of the head, delimited by the maxillary ridge on either side. 

 The floor, on the other hand, represents the mandibular ridge 

 (Fig. 80, H, m.r) which has grown forwards in a direction parallel 

 to the roof. The inner wall of the buccal cavity is in close contact 

 with the anterior extremity of the endodermal alimentary tube but 

 for a time the two cavities remain separated by a thin membranous 

 diaphragm made up of the apposed layers of ectoderm and endo- 

 derm. This may conveniently be termed the velar membrane as 

 the organ known as the velum in Amphioxus or Petromyzon consists 

 simply of the remains of this membrane. 



The formation of the anterior, stomodaeal, portion of the buccal 

 cavity is seen in its simplest form in some of the lower holoblastic 

 Vertebrates such as Crossopterygians, Lung - fishes or Urodele 

 amphibians. 



In Fig. 69, D (p. 126) in the case of Protopterus, or in Fig. 100 

 (p. 178) in the case of Polypterus (see also Fig. 80, H), what will 

 become later the anterior part of the buccal roof is seen to be 

 simply a portion of the ventral surface of the head, bounded behind 

 by the transverse mandibular ridge — the rudiment of the lower 

 jaw — and on each side by the longitudinal maxillary ridge. 



As is well shown in the figure of Polypterus, and as is also the 



VOL. II L 



