366 O^^' [Vol. I. 



Plate xiii. 



Figs. 7, A, 8, A, 9, A. — Sections through the head of the embryo figured 

 at I, A, cut transversely to the main long axis of the embryo. Fig. 7, A, shows 

 the medullary folds (J/F), meeting to enclose the cavity of the mid-brain, 

 o, opening to the primary first ventricle. Fig. 8, A. — Section passing through 

 the middle height of the canal connecting the primary first ventricle {Iv') 

 with the ventricle of the mid-brain {IIv). F/, epiblast; d, dorsal point of 

 union of the medullary folds. Fig. 9, A. — Section passing through the floor 

 of the canal connecting the primary ventricle of the fore -brain with that of 

 mid-brain. Op, hollowed part of the wall of fore-brain, which becomes 

 later the optic vesicle. Mes, mesoblast. {Z. 4. A.) 



Figs. 10, A, 11, A, 12, A. — Series A, continued: sections in the same 

 plane. 10, A. — Section touching the ventral outer surface of the brain, 

 tangential to the curve of the ventral surface caused by the primary cranial 

 flexure. A, artery. 11, A, 12, A. — Sections passing through the anterior 

 curved portion of the notochord (iV). IIIv, ventricle of hind-brain. //C, 

 rudiment of head-cavity. (^Z. 4. A.) 



Figs. 13, A, 14, A, 15, A. — Series A, continued: sections in the same 

 plane, through the head in the region of the blind end of the alimentary canal 

 and the mouth. ^Z,, alimentary canal. iV, notochord. ^i^, unpaired anterior 

 medullary fold. yl;«, Amnion. £/, epiblast. 7/C, part of rudiment of head- 

 cavity fused with epiblast and wall of the alimentary canal. Hp/i, rudimentary 

 roof of hypophysis. Hyp, hypoblast. Mes, mesoblast. M.in, mouth in- 

 volution. So, somatic layer of mesoblast. Sp, splanchnic layer of meso- 

 blast. Ep, in Fig. 14, A, is the epidermis covering externally the anterior 

 medullary fold; it is also the proximal wall of the mouth involution; the distal 

 wall of the mouth involution is formed by the amnion folds Am' (Fig. 14, A) 

 bending outward and forward, and finally meeting, as in Fig. 15, A, about Am'. 

 (Z. 4- A.) 



Figs. 16, A, 17, A, 18, A. — Series A, continued: sections in the same 

 plane, through the regions of the heart (HT), of the opening of the head 

 intestine to the yolk-sack, and of the middle of the back. CC, canalis cen- 

 tralis of spinal cord. Other letters the same as in preceding figures. 

 (Z. 4. A.) 



Fig. 19, B. — Series B is cut transversely to the long axis of the body. The 

 embryo is older than that of series A; the embryo figured in series B having 

 nine protovertebrte. The section (19, B) passes through the anterior curved 

 part of the notochord {IV), and through the head-cavities {HC), whose walls 

 (at CZ) are prolonged towards the anterior end of the notochord. HB, 

 hind-brain. FB, fore-brain. A, artery. (Z. 2. A.) 



Fig. 20, B. — Section through the anterior end of the alimentary canal 

 (ylZ,). Other lettei-s the same as in 19 B and the rest of the series. 

 (Z. 2. A.) 



Figs. 21, B, 22, B, 23, B. — Sections in the region of the first visceral 

 (hyoid) cleft (/). Ed, epidermis forming the roof of the mouth involution. 

 Ilph, the point where the hypophysis is formed. M, hypoblastic mouth. 

 M.in, epidermoidal mouth involution between fore-bi'ain and lower jaw. 

 E, ear. (Z. 2. A.) 



