TRANSVERSE SECTIONS OF A TEN MM. PIG EMBRYO 



135 



thelium. The first, second and third branchial arches show on either side of the section, the 

 third being slightly sunken in the cervical sinus. The dorsal diverticula of the third pharyngeal 

 pouches extend toward the ectoderm of the third branchial cleft. The ventral diverticula or 

 thymic anlages may be traced caudad in the series. The floor of the pharynx is sectioned 

 through the epiglottis. Ventral to the pharynx are sections of the third aortic arches and the 

 solid cords of the median thyreoid gland. Dorsally the section passes through the spinal cord and 

 first pair of cervical ganglia. Between the cord and pharynx, named in order, are the internal 

 jugular veins, the hypoglossal nerve, and the nodose ganglion of the vagus. Lateral to the ganglion 



N.accessonus 



Cang.Froriep 

 Myelencephahn 



Neural cavity 



Hoots of n. 

 hypoglossus 



Int. jugular vein 

 N.n. vagus et 

 accessorius 



Descend Aorta 



!\l. facialis 

 Br. arch 2 

 Tongue 

 Mandible 



Maxillary 

 process 



Telencephalon 



Fig. 125.— Transverse section passing through the first and second pharyngeal pouches of a 10 mm. pig 



embryo. X 22.5. 



is the external branch of the n. accessorius, and mesial to the ganglia are the small descending 

 aorta. 



Section through the Fourth Pharyngeal Pouches (Fig. 127).— This region is 

 marked by the disappearance of the head and the appearance of the heart in the pericardial 

 cavity. The tips of the atria are sectioned as they project on either side of the bulbus cordis. 

 The bulbus is divided into the aorta and pulmonary artery, the latter connected with the right 

 ventricle, which has spongy muscular walls. The pharynx is crescentic and continued laterally 

 as the small fourth pharyngeal pouches. Into the mid-ventral wall of the pharynx opens the 

 vertical slit of the trachea. A section of the vagus complex is located between the descending 



