364 ORR. , [Vol. I. 



Plate xii. 



Fig. I, a. — Young embryo viewed from beneath, in which the medullary 

 folds {MF') have coalesced only in the dorsal region. The tail (Z") is broken 

 at X. Jv^i primary first ventricle. MB, mid-brain, N, notochord. PV, pro 

 tovertebrse. Ales, unsegmented mesoblast. 



Fig. 2, C. — Embryo much more advanced than Fig. i, A. FB, MB, HB, 

 fore-, mid-, and hind-brain. HT, heart. Ey, eye. HC, head-cavity. Hp/i, 

 hypophysis. NM, neuromeres of the hind-brain. F, ear. /, //, ///, hyoid 

 and two succeeding gill-clefts. M, mouth. P-S, Plane of section for series 

 of figures 24, C — 44, C. 



Fig. 3 and Fig. 4. — Dorsal and side view of head of an embryo, showing 

 secondary cranial flexure in the hind-brain with the concomitant distending 

 of the walls. HB, hind-brain. E, ear. TH, thalamencepalon. 



Fig. 5. — Left wall of the medullary tube in the region of the fifth and sixth 

 nerves, shown in longitudinal-horizontal section. nV\, 2, 3> not of the trigem- 

 inus nerve. NM, neuromeres. Spf, lines of division between the neuro- 

 meres. (These lines have not been strongly enough marked in the plate.) 

 (Z. 2. D.) 



Fig. 6. — Horizontal section dorsal and parallel to the axis of the primary 

 fore-brain; showing the neuromeres {NM) of the thalamencephalon {TH). 

 FB", secondary fore-brain. il/Z>, Mid-brain. This embryo is further ad- 

 vanced than that of series C. {Z. 2. A.") 



