TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 183 



will continue through later stages until a new venous path is established on the 

 outside of the trigeminal ganglion, which will then appear, so to speak, as an 

 island in the path of the jugular. Still later the vein on the inner side of the tri- 

 geminal ganglion will abort, and only the external pathway will be retained. By 

 these changes the vein migrates from its original internal position to its new per- 

 manent external position. In the 1 2 mm. pig the jugular vein pursues a very sin- 

 uous course along the sides of the hind-brain, for it passes inside of the twelfth, 

 eleventh, tenth, and ninth nerves, then outside of the otocyst, the seventh and 

 eighth nerves, and inside of the fifth. In an earlier stage it lay inside of the oto- 

 cyst, but is now found to have migrated to the outside thereof, by the process of 

 island formation just described for the trigeminus. The blood-vessel in its origi- 

 nal condition is properly termed the anterior cardinal. When it has completely 

 migrated outside, not only of the otocyst and of the trigeminal ganglion, but also 

 outside of the glosso-pharyngeal and vagus nerves, it may properly be termed 

 the jugular. The jugular, therefore, is to be defined as the anterior cardinal vein 

 which, by successive island formations, has migrated to a new position outside of 

 the otocyst and cephalic ganglia. Third, to show the infundibular gland, 7n/, a 

 small evagination from the ventral floor of the fore-brain, F .h. The evagination 

 is really hollow, but the cavity does not appear in the section figured. It enters 

 into very close relations with another hollow evagination, which springs from the 

 dorsal roof of the oral cavity and is known as the hypophysis. The infundibu- 

 lar gland and the hypophysis become intimately associated with one another in 

 their further development and give rise to the pituitary body of the adult, the 

 gland becoming the posterior lobe, the hypophysis the anterior lobe of that organ. 

 The hypophysis may be best studied in sagittal sections (see page 205). The 

 present section, figure 115, being at a lower level than figure 113, passes through 

 the ventral portion of the hind-brain and shows only a narrow part of the cavity 

 of the fourth ventricle, Ven.iv. The three layers in the wall, Md, of the hind-brain 

 are very distinct. At the anterior end of the hind-brain appears a series of light 

 lines, Str, which are caused by nerve-fibers. These lines have been identified as 

 the stricB acusticce. They need to be more accurately studied, however, for they 

 seem rather to be fibers of the lateral root of the facial nerve. Close to the ante- 

 rior section of the jugular vein, Jug\ appear the minute fourth and third nerves, 

 which, however, are not indicated in the figure. Both lie close to the wall of the 

 vein on the side away from the trigeminal ganglion. The fourth nerve lies 

 nearer the outside of the embryo, the third nerve nearer the median plane. 

 At about the same level as this part of the jugular vein, and very close to the 

 wall of the fore-brain, is situated the loop of the internal carotid. Lower down, 

 but not close to the wall of the fore-brain, is the section of the lateral jugular. 



