Il6 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



The trochlear nerve fibers, motor, to the superior obHque muscle of the eye, arise 

 from the ventral wall of the mesencephalon, turn dorsad and cross at the isthmus, 

 thus emerging on the opposite side. From the myelencephalon arise in order 

 (5) the trigeminal nerve, mixed, with its semilunar ganglion and three branches, 

 the ophthalmic, maxillary, and mandibular; (6) the n. abducens, motor, from the 

 ventral wall to the external rectus muscle of the eye; (7) the n. facialis, mixed, 

 with its geniculate ganglion and its chorda tympani, facial, and superficial petrosal 

 branches in the order named; (8) the n. acusticus, sensory, arising cranial to the 

 otocyst, with its acoustic ganglion and sensory fibers to the internal ear; (9)' 

 caudal to the otocyst the n. glossopharyngeus , mixed, with its superior and petrosal 

 ganglia; (10) the vagus, sensory, with its jugular and nodose ganglia; (11) accom- 

 panying the vagus the motor fibers of the spinal accessory which take origin 

 between the jugular and sixth cervical ganglia from the lateral wall of the spinal 

 cord and myelencephalon; the internal branch of the n. accessorius accompanies 

 the vagus; the external branch leaves it between the jugular and nodose ganglia 

 and suppKes the sternocleidomastoid and trapezius muscles; (12) the n. hypoglos- 

 sus, motor, arising by five or six fascicles from the ventral wall of the myelen- 

 cephalon; its trunk passes lateral to the nodose ganglion and supphes the muscles 

 of the tongue. 



A nodular chain of ganglion cells extends caudad from the jugular ganglion of the vagus. 

 These have been interpreted as accessory vagus ganglia. They may, however, be continuous 

 with Froriep's ganglion which sends sensory fibers to the n. hypoglossus. In pig embryos of 

 15 to 16 mm. this chain is frequently divided into four or five gangUonic masses, of which 

 occasionally two or three (including Froriep's ganglion) may send fibers to the root fascicles 

 of the hypoglossal nerve. Such a condition is shown in Fig. 121. 



Spinal Nerves. — These have each their spinal ganglion, from which the dorsal 

 root fibers are developed (Figs. 120 and 136). The motor fibers take origin from 

 the ventral cells of the neural tube and form the ventral roots which join the 

 dorsal roots in the nerve trunk. 



In Fig. 120 the heart with its right atrium and ventricle, the dorsal and ven- 

 tral lobes of the liver, and the large mesonephros are prominent. Dorsal and 

 somewhat caudal to the atrium is the anlage of the right lung. The septum 

 transversum extends between the heart and the liver. 



Pharynx and its Derivatives.— Dorsally the anterior lobe of the hypophysis 

 is long and forks at its end (Figs. 122 and 123). In the floor of the pharynx are 

 the anlages of the tongue and epiglottis (Fig. 156 ^). From each mandibular arch 

 arises an elongated thickening which extends caudal to the second arch. Be- 



