xvi EXPLANATION OF PLATES 8 AND 9. 



10 a. The most posterior of the three sections. It shews the posterior flatness 

 of the medullary groove and the two isolated vertebral plates. 



io. This section is taken from the anterior part of the same embryo and 

 shews the deep medullary groove and the commencing formation of the ventral wall 

 of the alimentary canal from the nuclei of the yolk. 



10 c shews the disappearance of the medullary groove and the thinning out of 

 the mesoblast plates in the region of the head. 



Fig. 1 1. Small portion of the blastoderm and the subjacent yolk of an embryo at 

 the time of the first appearance of the medullary groove x 300. It shews two large 

 nuclei of the yolk (n) and the protoplasmic network in the yolk between them ; the 

 network is seen to be closer round the nuclei than in the intervening space. There 

 are no areas representing cells around the nuclei. 



Fig. i?. Nucleus of the yolk in connection with the protoplasmic network 

 hardened in osmic acid. 



Fig. 13. Portion of posterior end of a blastoderm of stage B, shewing the forma- 

 tion of cells around the nuclei of the yolk. 



Fig. 14. Section through part of a young Scyllium egg, about T Vth of an inch in 

 diameter. 



nl. Protoplasmic network in yolk. zp. Zona pellucida. ch. Structureless 

 chorion. fep. Follicular epithelium, x. Structureless membrane external to this. 



EXPLANATION OF PLATES 8 AND 9. (X. p. 286.) 

 COMPLETE LIST OF REFERENCE LETTERS. 



a. Arteries of yolk sac (red), al. Alimentary cavity, alv. Alimentary vesicle 

 at the posterior end of the alimentary canal, an. Point where anus will appear. 

 au v. Auditory vesicle. bL Blastoderm, ch. Notochord. e s. Embryo-swelling, h. 

 Head. ht. Heart, m. Mouth, m g. Medullary groove, mp. Muscle-plate or proto : 

 vertebra, op. Eye. s c. Segmentation cavity, sos. Somatic stalk, t s. Tail-swelling. 

 v. Veins of yolk sac (blue), vc. Visceral cleft. I. vc. ist visceral cleft, .r. Portion 

 of blastoderm outside the arterial circle in which no blood-vessels are present 

 yk. Yolk. 



PLATE 8. 



Fig. A. Surface view of blastoderm of Pristiurus hardened in chromic acid. 

 Fig. B. Surface view of fresh blastoderm of Pristiurus. 

 Figs. C, D, E, and F. Pristiurus embryos hardened in chromic acid. 

 Fig. G. Torpedo embryo viewed as a transparent object. 

 Figs. H, I. Pristiurus embryos viewed as transparent objects. 

 Fig. K. Pristiurus embryo hardened in chromic acid. 



The remainder of the figures are representations of embryos of Scyllium canicula 

 iened in chromic acid. In every case, with the exception of the figures marked P 

 i Q, two representations of the same embryo are given ; one from the side and one 

 from the under surface. 



