372 ORE. [Vol. I. 



Plate xvi. 



Fig. 59, B. — Transverse section of the young embryo of series B, showing 

 the heart {HT) and the first formation of the ear (jE). AL, alimentary 

 canal. {Z. 2. A.) 



Fig. 60. — Section at right angles to the anterior surface of the fore-brain, 

 through the eye and optic stalk of an embryo much more advanced than that 

 in Fig. 2, C. Ey', inner wall of the eye-cup. Ey", outer wall of the eye-cup, 

 A, artery entering the choroid slit. Op", anterior wall of optic stalk, with 

 nerve-fibres {NF) which run into the inner surface of the eye-cup. Op', pos- 

 terior wall of optic stalk. /, lumen of optic stalk. JFB, wall of fore-brain. 

 {Z. 4- A.) 



Fig. 61. — Section in the same plane as Fig. 60, through the optic stalk 

 where its lumen communicates with that of the fore-brain. ivFB, wall of 

 fore-brain. For other letters see Fig 60. {^Z. 4. A.) 



Figs. 62, F, 63, F, 64, F. — Slightly oblique sagittal sections through the 

 head of an embryo in which the paired lobes of the secondary fore-brain 

 (^HM) have appeared. Section 62, F, passes through the right lateral wall of 

 the brain, cutting through the ventricles where they are most distended lat- 

 erally. Section 63, F, shows the connection of all the ventricles. A line 

 drawn through the hypophysis and posterior commissure would lie in the 

 median sagittal plane of the embryo and in the plane of section; that part 

 anterior to this imaginary line diverges very slightly to the right of the me- 

 dian plane. Fig. 64, F, represents, more highly magnified, the optic region 

 of section 63, F. Ch, chiasma. Ep, epidermis. HB, hind-brain. HM, 

 hemispheres. LF, longitudinal nerve-fibres. LJ, lower jaw. oe, cesopha- 

 gus. PCs, posterior commissure. Hph, hypophysis. In, infundibulum. 

 TH, thalamencephalon. 62, F, 63, F (Z. 2, Obj. 2 inch, Ross), 64, F. 

 (Z. 4. A.-) 



Fig. 65. — Transverse section through the ventral wall of the mid-brain 

 {MB), showing the exit of the third pair of nei-ves {nlll). LF, longitudinal 

 fibres. TF, transverse fibres. {Z. 4. A.) 



Figs. 66 and 67. — Transverse sections through the spinal medulla at an 

 earlier (66) and a later (67) stage of development, ar, anterior nerve-root. 

 CC, canalis centralis. dL, dorsal longitudinal fibres. Ept, epithelial floor of 

 canalis centralis, g, ganglion of posterior nerve-root, gc, anterior gray 

 column. Mp, muscle-plate. N, notochord. pt, ventral downgrowth of the 

 lateral walls of the medulla, which enclose later the anterior fissure. TF^ 

 transverse fibres, which unite with the prolonged distal ends of the nerve- 

 cells in the region marked y. vL, ventral longitudinal fibres. (The ventral 

 longitudinal fibres in Fig. 66 are not quite so conspicuous in the plate as they 

 are in the section.) {Z. 4. AS) 



