186 STUDY OF PIG EMBRYOS. 



lateral roots. The positions of the exits of these two bundles of nerve-fibers are 

 constant and characteristic. The ventral root always passes out from the middle 

 of the ventral zone about half-way between the median floor-plate and the dorsal 

 limit of the zone. The lateral root always passes out at the upper dorsal limit of 

 the ventral zone and immediately below the point of entrance of the true dorsal 

 root. Formerly the lateral roots were not distinguished from the dorsal roots. 

 Following downward in the figure we come to the section of the jugular vein, Jug, 

 just inside of which lies the common trunk, N. 10. //, of the united tenth and 

 eleventh or vagus and accessorius nerves, and also, nearby, the lower part of the 

 petrosal ganglion, TV. p. petr, of the glosso-pharyngeal nerve. Lower down and 

 nearer the ectoderm lies the facial nerve, N. 7, situated in what is called the hyoid 

 arch or mass of tissue intervening between the first and second gill cleft. The 

 hyoid arch is further marked by a bulge in the external outline of the section, 

 which leads down into a deep groove beyond which the outline of the section 

 again rises and arches forward to the eye. This groove is the external depres- 

 sion of the first gill cleft and ultimately is transformed into the external auditory 

 meatus. The position of this groove is well shown in figure 99, Au, on page 160. 

 Just inside the auditory groove appears the outer end of the first or audi- 

 tory internal gill pouch, cl. I. It is a long, oblique slit, quite narrow, and is 

 lined by a layer of entoderm.^' If we follow it along through several sections, we 

 shall find that higher up its outer end comes in contact with the ectoderm at 

 the bottom of the auditory groove, and there the two germ-layers, entoderm 

 and ectoderm, unite to form a single membrane, the closing plate of the gill 

 pouch. Following through the section downward in the series, we can trace 

 the cleft to its connection with the pharynx, Ph. On the posterior side of 

 the cleft we find the internal carotid, Car. in. Only the roof of the pharynx, 

 Ph, is cut, so that it occupies but a small area in the section. On its anterior 

 side it shows a small knob-like projection toward the floor of the fore- 

 brain. This is a part of the stalk of the hypophysis. Below the hypophysis 

 appears the very large and conspicuous inferior maxillary nerve, MX. in, and 

 beneath that the section of the small internal jugular vein, Jug. in. The fore- 

 brain is quite complicated in shape, having two lateral expansions, L. V, of its 

 cavity which are destined to form the lateral ventricles. The walls, H, of the 

 lateral ventricles are the anlages of the cerebral hemispheres. From the ventral 

 (in the figure upper) part of the fore-brain spring on either side the optic stalks, 

 Op. These are hollow prolongations of the brain, which expand at their distal 

 ends to form the retina of the eye, Ret, and the pigment layer. The expanded 

 distal ends constitute each a sort of cup, of which the optic stalk is the stem. The 

 cup is two-layered, the space between the two layers being a prolongation of the 





