3/0 ORR. ' [Vol. I. 



Figs. 48, D, 49, D, 50, D. — Three sections, cut nearly in a sagittal plane, 

 of an embryo slightly younger than that of series C. 



Fig. 48, D, cuts the optic stalk {O.st) outside of the fore-brain; shows the 

 mandibular arch {A) \ also the head-cavity ; and passes through the median 

 vertical plane at the thin points of the brain-roof where the mid-brain joins 

 the fore-brain on one side and the hind-brain on the other. Fig. 49, D, 

 passes through the median vertical plane at the point where the notochord 

 touches the infundibular region of the fore-brain {FB). Fig. 50, D, passes 

 through the median vertical plane at the extreme anterior dorsal region of the 

 fore-brain, and also at a point low down in the hind-brain (HB). I, dorsal 

 end of the hyoid-cleft. 00; optic groove. For the other parts see Register 

 of Index-letters. {Z. 2. A.) 



Fig. 51, D. Diagrammatic median sagittal section of the embryo repre- 

 sented in series D (reconstructed from 22 drawings of consecutive sections). 

 This figure shows the ventricles of the fore-, mid-, and hind-brain (FB, MB, 

 I/B), the first two separated by a lateral constriction posterior to c. It also 

 illustrates the primary cranial flexure, the position of the notochord (iV), and 

 the first rudiment of the thyroid gland (T/i), 



Fig. 52, E. — Section passing through the anterior-dorsal wall of the mid- 

 brain {MB) forward through the anterior wall of the primary fore-brain {FB) 

 at right angles, immediately ventral to the eye-stalks. It shows nerve-fibres 

 belonging to the main system of longitudinal fibres (LF) uniting anteriorly 

 (LF"). Dorsal-wards are seen in oblique section the fibres of the posterior 

 commissui-e (PCs). (Z. 2. A.) 



Figs. 53, E, 54, E. — Sections in the same plane as 52, E, cut a little farther 

 forwards. Fig. 53, E, shows the posterior commissure (FCs) where its 

 fibres pass around the dorsal summit of the brain, immediately in front of the 

 mid-brain. Fig. 54, E, shows the first rudiment of the epiphysis (Eph). 

 iZ.2. A.) 



Fig. 55. — Transverse section through the primary fore-brain, passing 

 through the hypophysis (HfJi) and the optic stalks {O.st). LF, cross-sec- 

 tion of longitudinal fibres passing ventral to the eye-stalks. {Z. 2. A^ 



Figs. 56, 57, 58. — Transverse sections through the tail of an advanced em- 

 bryo. Fig. 56 shows a section about the middle of the tail. Fig. 57, a section 

 near the tip, and Fig. 58, a section just at the tip. A, artery. Md, medulla. 

 N, notochord (in 58 remarkably pushed away from its usual central position). 

 AL, rudiment of the caudal intestine, vv, veins, mf, muscle somites. Mes, 

 mesoblast. MHp, indistinguishable mass of mesoblast and hypoblast. Ne.c, 

 neurenteric canal. {Z. 4. A.) 



