Il8 THE STUDY OF SIX AND TEN MILLIMETER PIG EMBRYOS 



nerve is represented by the optic stalk cut through in Fig. 120. (3) The 

 oculomotor, a motor nerve to four of the eye muscles, takes origin from the 

 ventro-lateral wall of the mesencephalon. (4) The trochlear nerve fibers, 

 motor, to the superior oblique 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 n. trigeminus, 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 n. vagus, sensory, 

 with its jugular and nodose ganglia; (n) the n. accessorius, whose motor 

 fibers take origin from the lateral wall of the spinal cord and myelencepha- 

 lon between the jugular and sixth cervical ganglia; the internal branch of 

 the n. accessorius accompanies the vagus; the external branch leaves it 

 between the jugular and nodose ganglia and supplies the sternocleidomas- 

 toid and trapezius muscles; (12) the n. hypoglossus, motor, arising by five 

 or six fascicles from the ventral wall of the myelencephalon; its trunk 

 passes lateral to the nodose ganglion and supplies 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 gan- 

 glionic 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. 



The Spinal Nerves. Each of these has its own 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. 



Besides the nervous system, Fig. 120 also shows the heart, with its 

 right atrium and ventricle, the dorsal and ventral lobes of the liver, and the 

 prominent mesonephros. 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 



