EMBRYOLOGY. 273 



Face and Visceral Arches. In the facial and cervical regions 

 the visceral laminae send up three processes, the visceral arches, 

 separated by clefts, the visceral clefts. 



The -first, or the mandibular arches, unite in the median line to 

 form the lower jaw, and superiorly form the malleus. A process 

 jutting" from its base grows forward, unites with the frontonasal 

 process growing from above, and forms the upper jaw. When the 

 superior maxillary processes fail to unite there results the cleft- 

 palate deformity ; if the integument also fails to unite there results 

 the hare-lip deformity. The space above the mandibular arch becomes 

 the mouth. 



The second arch develops the incus and stapes bones, the styloid 

 process and ligament, and the lesser cornu of the hyoid bone. The 

 cleft between the first and second arches partially closes up, but 

 there remains an opening at the side, which becomes the Eustachian 

 tube, tympanic cavity, and external auditory meatus. 



The third arch develops the body and greater cornu of the hyoid 

 bone. 



Alimentary Canal and Its Appendages. The alimentary canal is 

 formed by a pinching-off of the yolk-sac by the visceral plates as they 

 grow downward and forward. It consists of three distinct portions 

 the fore gut, the hind gut, and the central part, which communi- 

 cates for some time with the yolk-sac. It is at first a straight tube, 

 closed at both extremities, lying just beneath the vertebral column. 

 The canal gradually increases in length and becomes more or less 

 convoluted ; at its anterior portion two pouches appear, which be- 

 come the cardiac and pyloric extremities of the stomach. At about 

 the seventh week the inferior extremity of the intestine is brought 

 into communication with the exterior by an opening, the anus. An- 

 teriorly the mouth and pharynx are formed by an involution of epi- 

 blast, which deepens until it communicates with the fore gut. 



The liver appears as a slight protrusion from the sides of the ali- 

 mentary canal, about the end of the first month ; it grows very rapidly, 

 attains a large size, and almost fills up the abdominal cavity. The 

 hepatic cells are derived from the intestinal epithelium, the vessels 

 and connective tissue from the mesoblast. 



The pancreas is formed by the hypoblastic membrane. It origi- 

 nates in two small ducts budding from the duodenum, which divide 

 and subdivide, and develop the glandular structure. 

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