336 ANATOMY OF THE DOMESTIC FOWL 



the cranial flexure, and is united- to the hindbrain by a narrow 

 isthmus. The metencephalon, or rudimentary cerebellum, appears 

 as a thick portion on the most anterior division of the hindbrain. 

 The rest of the hindbrain is provided with a transparent roof and 

 constitutes the myelencephalon, or the rudimentary medulla ob- 

 longata. Observe the inner and the outer wall of the optic cup, the 

 lens, the choroid fissure, and the posterior, or vitreous, chamber. 



Note the form of the otocyst, or auditory sac. Note above which 

 visceral arch it lies. Note the number of visceral clefts. The 

 visceral arches are formed by the thickening of the walls of the 

 bounding clefts. The visceral arches are as follows: the first is 

 the mandibular, or hyomandibular, arch, which is in front of the 

 first cleft. From this there is developed the lower jaw. Note if 

 there is a maxillary process arising from the dorsal angle of the 

 arch. The second arch is the hyoid arch, which is located behind 

 the first cleft. Then follow in order the third, the fourth, and the 

 fifth visceral arches. Note above the mandibular arch the rudi- 

 mentary trigeminal ganglion, and above the hyoid arch the rudi- 

 mentary acoustico-facialis. The latter is in contact with the anterior 

 walls of the auditory sac. The rudimentary glossopharyngeal 

 ganglion is noted above the third visceral arch. The vagus, or 

 pneumogastric, ganglion is located above the fourth and the fifth. 

 Note the form and the postion of the heart. Note the anterior and 

 the posterior limb rudiment in the trunk. 



In a study of the sections it is found that cuts transverse to the 

 trunk pass about horizontally through the forebrain. At this age 

 the following sections of the embryo should be studied: 



First, through the hindbrain, at which level will be noted the 

 auditory sacs, the neuromeres, the trigeminal, the acoustico-facialis, 

 and the glossophar3Tigea] and the vagus ganglion. 



Second, through the upper part of the pharynx, at which level 

 wUl be observed the midbrain, the hindbrain, the visceral pouches, 

 the nerves, and the blood-vessels. 



Third, through the choroid fissure of the optic cups. Note the 

 parts of the eye, and, on the other side of the section, the heart. 



Fourth, a study of a section through the olfactory pits. 



Fifth, a study of a section through the pancreatic and the hepatic 

 dverticula. 



Sixth, a study at the beginning of the allantois through the hind- 

 gut. 



