77i DEYELOPMEXT OF THE ALIMEXTAEY CAXAL. 



formed from peripheral blastema like all other n-erves, with the exception of the 

 optic. 



DEVELOPMENT OF THE ALIMENTAEY CANAL AND ORGANS ARISING 

 FROM THE HYPOBLAST. 



The whole alimentary canal, from the fauces to the anus, together 

 with the rudiments of certain organs associated with it in their com- 

 mencement, viz., the thyroid gland, lungs, trachea and larynx, the liver, 

 and pancreas, as well as the allantois, owe their origin more imme- 

 diately to inflections of the hypoblast layer of the germinal membrane, 

 which supplies the epithelial lining of their principal cavities ; but in 

 all these organs parts of their structure are supplemented, and some 

 other organs, such as the mesentery and spleen, are wholly formed from 

 the mesoblast, whence proceed the vascular, fibrous, and parenchyma- 

 tous elements, and also the serous coverings of the organs, where these 

 exist. 



ALIMENTARY CANAL, 



The primary digestive cavity of birds and mammals, as it extends 

 from one end of the embryo to the other below the vertebral axis, 

 presents at first a manifest division into three parts. One of these^ 

 occupying the part of the embryo which is enclosed by the cephalic 

 fold, and which may be named the foregut, comprises the rudiments 

 of the pharynx and gullet, the stomach and duodenum. The posterior 

 division, which is comparatively short, occupies the caudal fold of the 

 embryo, and corresponds mainly to the lower part of the colon and 

 rectum. Both of these parts have from the first a tubular form, and 

 are closed respectively by the Inflection of the whole blastodermic 

 layers at the anterior and posterior extremities of the body. The 

 middle division has primarily the form of a long and wide groove, 

 lying close below the corresponding part of the vertei)ral bodies, lead- 

 ing at its opposite ends into the cephalic and caudal portions of the 

 gut, and is freely open throughout on its ventral aspect into the cavity 

 of the yolk-sac, with the blastodermic walls of which, as formerly 

 described, the constituents of the intestinal walls are directly con- 

 tinuous (see fig. 576). 



The mouth, as elsewnere explained, is no part of the primitive 

 alimentary canal, but is formed by involution of parts of the face, and 

 receives, therefore, its lining membrane from epiblast. It is separated 

 for a time from the pharynx, which is the foremost part of the primi- 

 tive alimentary canal, by the reflection of the layers of the blastoderm, 

 and the communication which is later established between the mouth 

 and pharynx at the posterior arch of the fauces, is due to a solution of 

 continuity in these layers, which occurs in the chick on the fourth day 

 of incubation, and has been traced at a corresponding period of 

 development in several mammals. The aperture has at first the form 

 of a vertical slit, which widens later as it becomes the opening from 

 the pharynx into the common cavity of the nose and mouth. The 

 diverticulum of the pituitary gland, it will be remembered, occupying- 

 the place which becomes the top of the permanent pharynx, is formed 

 in connection with the epiblastic or buccal, and not the hypoblastic or 

 pharyngeal division of the alimentary passage (see fig. 535, A and B, py). 



The hypoblastic layer of the germinal membrane, from which is 

 derived the epithelial lining of the whole alimentary canal and pas- 



