THE DIGESTIVE TRACT. lg 



as in the case of the salivary glands, is still undetermined; the 

 fibrillae are traceable to the basement-membrane of the acini, but 

 their further accurate disposition remains undecided. 



The development of the digestive tract and its appendages in- 

 volves all three blastodermic layers, the mesoderm and the ento- 

 derm, however, being the ones participating to the greatest extent. 

 The epithelium of the mucous membrane, together with that of the 

 glandular structures connected therewith, is the direct derivative of 

 the entoderm, with the exception of that lining the oral cavity ante- 

 rior to the fauces and the salivary and oral glands, the epithelium of 

 which parts originates from the ectodermic invagination. For a short 

 distance within the anus, likewise, the ectoderm contributes the cells 

 lining the gut. As already pointed out, the enamel and the dentine 

 are also products respectively of the ectoderm and of the mesoderm. 



The formation of the gut-tract consists essentially of a process 

 of folding off and closing 



together of the ventral body- FIG. 232. 



plates, which are composed 

 of the entoderm united with 

 the visceral layer of the meso- 

 derm. The tube thus formed 

 begins in the cephalic region 

 of the embryo as a blind, 

 somewhat dilated pouch, the 

 primitive pharynx, which 



for a short time is Separated Transverse section of nine-day rabbit embryo, show- 



r i 1 ing formation of primitive gut (g) by approximation of 



irom tne primary Oral reCeSS, ventral plates composed of visceral layers of mesoderm 



Or StOmodaeum, by a parti- and entoderm (<); m, m, body-cavity bounded by 



tion, the pharyngeal plate, and visceral she 



consisting of the opposed 



ectoderm and entoderm ; after the rupture of this plate the gut-tract 

 communicates directly 'with the exterior through the oral cavity. A 

 somewhat similar process takes place at the lower part of the primitive 

 digestive tube, whereby the anus becomes established. For a con- 

 siderable time the gut communicates with the cavity of the umbilical 

 vesicle through its duct. The several divisions of the primary diges- 

 tive tube, its wall consisting of epithelial lining and supplementary 

 mesodermic tissue, undergo differentiation and acquire distinctive 

 characters, which, however, depend largely upon the differentiation 

 of the embryonal epithelial layer. 



The division of the tube into particular regions begins with the 

 stomach, which as early as the fourth week in the human embryo 

 is distinguishable as a spindle-shaped enlargement. With the sub- 

 sequent rapid increase in the size of the organ, the tissues constituting 



