THE PEIMITIVE ALIMENTAEY CANAL. 



43 



The first of the pharyngeal bars is the rudiment of the maxillary and mandibular 

 regions. It is called the mandibular arch. The second is the hyoid arch, and the 

 remainder are the branchial arches proper. 



When they first appear, the arches extend from the level of the dorsal wall of 

 the fore-gut to the pericardium but, as growth proceeds, and the neck is developed 

 between the head and the pericardium, the ventral ends of the arches of opposite 

 sides meet in the ventral wall of the primitive pharynx. The growth of the mandi- 



Rudiment of respiratory system 



Ectoderm of embryo | 



Ectoderm of amnion 



Mesoderm of amnion 



Thyreo-glossal d 



Hind-brai 



Medulla spinalis 



Notochord 



t Dorsal pancreas rudiment 



Peritoneal part of coelom 



Seessel's pouch 



Mid- brain 



Peritoneal part of 

 coelom 



loaca 



Rathke's pouch 



Cerebral hemisphere 



Pericardium / 



Rudiment of liver | 



Septum transversum 



Rudiment of gall-bladder 

 Ventral pancreas rudiment 



Vitello-intestinal duct 



FIG. 57. SCHEMA OF A LONGITUDINAL SECTION OP AN EMBRYO. (After Mall, modified, 

 dorsal and ventral divertricula for alimentary canal. The heart is not shown. 



oacal membrane 



'ail-gut diverticulum 

 horion 



Allan toic 

 diverticulum 



Showing 



bular and the hyoid arches soon greatly exceeds that of the branchial arches proper, 

 and the latter gradually recede from the surface until, on each side, they lie at the 

 bottom of a depression, the precervical sinus, which is overlapped by the caudal 

 border of the hyoid arch. As the overgrowth of the hyoid arch continues the open- 

 ing of the precervical sinus to the surface is reduced to a narrow channel, the precer- 

 vical duct. Afterwards this is obliterated, the sinus becomes the precervical vesicle, 



1st cleft ' 



2nd cleft- 



cleft " 



Precervical duct' 



4th cleft ' 



Branchial duct 

 Precervical_ 

 sinus 



Precervical 

 duct 

 4th pouch 



FIG. 58. SCHEMA showing the formation of the precervical sinus, the branchial ducts, and the 



precervical sulcus. 



but the position of the original aperture of the precervical duct is temporarily 

 indicated by a sulcus, the precervical sulcus which soon disappears. The precervical 

 vesicle lies at the side of the third pharyngeal cleft, and it is associated with 

 the second and fourth clefts by narrow canals, the branchial ducts, which are the 

 remains of the branchial clefts. Ultimately the precervical vesicle and the branchial 

 ducts disappear, but it has been suggested that before the vesicle disappears a 

 part of the lobe of the thymus of the same side is formed from its wall. 



