DEVELOPMENT OF THE MOUTH. 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



The early development of the primitive alimentary canal has already been briefly 

 described in treating of the first formation of the embryo (pp. 34, 35), and it was there 

 explained how the dipping downwards and inwards of the blastodermic layers on 

 either side of the embryo tends to separate or pinch off the part of the blastodermic 

 vesicle which is immediately underneath the body of the embryo as a distinct tube 

 (mid-gut) from the remainder of the vesicle, which is now known as the yolk-sac, while 

 at the same time similar changes occurring in front- and behind produce the blind 

 anterior and posterior extremities of the tube which are known as the fore- and hind- 

 gut respectively. Although the downfolding in question eventually involves all the 

 layers of the embryonic blastoderm, the epiblast and the part of the mesoblast which 

 adheres to it, and together with it forms the splanchnopleure, do not participate in 

 the process until after the formation of the amnion, so that the alimentary canal for 

 some time after its formation is enclosed only by the hypoblast and its adherent 

 mesoblast (splanchnopleure), and projects freely into the wide ccelom or space 

 between the splanchnopleure and somatopleure. The mid-gut also remains for a 

 time in free communication with the yolk sac, although the communication becomes 

 gradually narrowed into the vitelline duct. As the somatopleure afterwards grows 

 down on either side of the alimentary canal, and becomes pinched in around the 

 vitelline duct and stalk of the allantois, which are thus united into the umbilical 

 cord, that part of the ccelom which is within the body and around the alimentary 

 canal becomes shut off as the pleuroperitoneal cavity from the remainder, which lies 

 altogether outside the body, and forms the cavity of the false amnion. 



Development of the mouth and of the parts in connection with it. 

 The fore-gut terminates blindly at first underneath the head in the region of the 



Fig. 116. FRONTAL VIEW OP THE UPPER PART OP A HUMAN 



EMBRYO OP ABOUT FIFTEEN BAYS, RECONSTRUCTED FROM 

 SERIAL SECTIONS. (His. ) <f 



The pericardium is opened to show the heart ; between this 

 and the fore-brain is seen the primitive biiccal cavity. A de- 

 scription of this figure is given on p. 138. 



hind-brain, and the notochord, with the fore- and 

 mid-brain, curve downwards over the blind extre- 

 mity, the fore-brain thus causing a rounded pro- 

 minence in front of and ventral to the extremity of 

 the alimentary tube (see fig. 45). "With the develop- 

 ment of the heart another prominence becomes 

 formed on the ventral side of the fore-gut, a little 

 further back. Between the two prominences, the 

 one caused by the projection of the fore-brain and 

 the other of the heart, a wide, shallow pit is enclosed 

 (fig. 116), at the bottom of which the epiblast 



which lines it is in contact with the hypoblast of the fore-gut, and the two layers 

 fuse to form an epithelial membrane, which now forms a septum between the 

 primitive buccal epiblastic involution or stomodseum and the fore-gut (fig. 117, 

 p.v.). This stage is met with in the human embryo before the twelfth day (His), 

 in the rabbit embryo at about the ninth day (Mihalkovics), and in the chick on the 

 fourth day. ..,,, , ,., ,y, , 



! V r'-"r .- 1 - H 2 



