THE ANATOMY OF A SIX MM. PIG EMBRYO 95 



ganglion nodosum of the vagus nerve; the ganglionic crest and the proximal 

 portion of the spinal accessory nerve; and the anlage of Froriep's ganglion, an 

 enlargement on the neural crest just cranial to the first cervical ganglion. 

 Between the vagus and Proriep's ganglion may be seen the numerous root 

 fascicles of the hypoglossal nerve, which take their origin along the ventro- 

 lateral wall of the myelencephalon and unite to form a single trunk. The 

 posterior roots of the spinal ganglia are very short; their anterior, or 

 ventral roots are not shown. 



The position of the heart with its ventricle, atrium, and sinus venosus 

 is shown. The liver is divided into a small dorsal and a large ventral 

 lobe. The fore-gut emerges from between the liver lobes and curves 

 ventrad to the yolk stalk and sac. The hind-gut is partly hidden by the 

 fore-gut; it makes a U-shaped bend from the yolk stalk to the caudal 

 region. The gut is attached to the dorsal body wall by a double layer 

 of splanchnic mesoderm which forms the mesentery. The long, slender 

 mesonephros lies ventral to the spinal cord and curves caudad from a point 

 opposite the eighth cervical ganglion to the tail region. The cranial 

 third of the mesonephros is widest and its size diminishes tailwards. 

 Between the yolk sac and the tail the allantois is seen, its stalk curving 

 around from the ventral side of the tail region. 



Digestive Canal/ -The arrangement of the viscera may be seen in 

 median sagittal and ventral dissections (Figs. 96 and 97), and also in the 

 reconstruction shown in Fig. 105. The mouth lies between the mandible, 

 the median frontonasal process of the head, and the maxillary processes 

 at the sides. The diverticulum of the hypophysis (Rathke's pouch), 

 flattened cephalo-caudad and expanded laterad, extends along the ventral 

 wall of the fore-brain (Fig. 105). Near its distal end, the wall of the brain 

 is thickened and later the posterior lobe of the hypophysis will develop 

 from the brain wall at this point. 



The pharynx is flattened dorso-ventrally and is widest near the mouth. 

 Its lateral dimension narrows caudad, and opposite the third branchial 

 arch it makes an abrupt bend, a bend which corresponds to the cervical 

 flexure of the embryo's body (Figs. 104 and 105). In the roof of the 

 pharynx, just caudal to Rathke's pouch, is the somewhat cone-shaped 

 pouch known as Seessel's pouch, which may be interpreted as the blind, 

 cephalic end of the fore-gut. The lateral and ventral walls of the pharynx 

 and oral cavity are shown in Fig. 98. Of the four arches the mandibular 

 is the largest and a groove partly separates the processes of the two sides. 

 Posterior to this groove and extending in the median plane to the hyoid 

 arch is a triangular elevation, the tuberculum impar; it later vanishes, 

 apparently contributing nothing to the tongue. At an earlier stage the 

 median thyreoid anlage grows out from the mid- ventral wall of the pharynx 



