TRANSVERSE SECTIONS OF EMBRl L 12 MM. 271 



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pharyngeal nerve. Lower down and nearer the ectoderm lies the facial nerve, 

 N.'j, situated in what is called the hyoid arch or mass of tissue intervening be- 

 tween the first and second gill-clefts. 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 depression of the first gill-cleft and ultimatejy is trans- 

 formed into the external auditory ifteatus. The position of this groove is well shown 

 in figure 166, Au, on page 223. Just inside the auditory groove appears the outer 

 end of the first or auditory 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 first gill-cleft appears the very 

 large and conspicuous inferior maxillary nerve, Mx.in, and beneath that the section 

 of the small ophthalmic vein, Op.v. 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 figu ' of the fore-brain 



spring on either side the optic stalks, Op. These arc hollow lions of the 



brain, which expand at their distal ends to form the retina of 

 pigment layer. The expanded distal ends constitute each a sort of cup, of^'J liVl 

 optic stalk is the stem. The cup is two-layered, the space between the two layers be- 

 ing a prolongation of the central cavity of the brain. The inner of 1 the two layers 

 forms the retina proper and is considerably thickened. The outer layer is quite thin 

 and is already quite abundantly laden with pigment granules. At .the edge of tb', 

 cup the pigment layer passes over uninterruptedly into the thick retina layer. Jn 

 the cavity of the optic cup lies the vesicular lens, L, which arose from an evagma- 

 tion of the overlying ectoderm. The vesicle is, however, now completely separated 

 from the layer which produces it. It has at this stage a very largei cavity, and in 

 cent sections it can be readily seen that the innc: side or that toward the brain 

 -eady thickening and changing its character so >;rn the main part of 



adult lens. The thickening depends chiefly upon the rapid and enyimon 

 of the epithelial cells of this part of the vesicle, so that they 



the so-called fibers of the adult K-ns, (. ndull 



Section through -id Gill-Cleft <. / Oral . c\cl oi 



ihis section is such that tn head is cut separately jnd appears in sermon without 



