NOTE ON THE DEVELOPMENT OF AMPHIBIANS. 323 
PLATE XXVII. 
Fig. 1. — Median-longitudinal and nearly vertical section of the egg of 
Amblystoma, at a time when the medullary plates have first appeared. It 
shows the anterior epiblastic thickening {a. M. P.), which unites anteriorly 
the two dorsal medullary plates. Also the thinner median portion {g. g.) 
between the dorsal medullary plates. Bl. Region of the blastopore. N. Un- 
differentiated hypoblastic tissue of the notochord. (Z. 2, A A.) 
Figs. 2 a, 3 b, 4 c, and 5 d. — T ransverse sections through the anterior 
dorsal region of embryos of Amblystoma, showing successive stages of develop- 
ment. The first of the series (2 a) is at a stage corresponding with that of 
Fig. 1. G. The thinner median portion of epiblast between the dorsal medul- 
lary plates which becomes pushed downwards, so that the surfaces immediately 
lateral to it become pressed together along the Hue g. (Z. 2, A A.) 
Figs. 6 a, 7 b, 8 c, 9 d. — Transverse sections through the anterior region of 
the head of the same embryos respectively as Figs. 2 a — 5 d. These sections 
show the anterior medullary plate {a. M. P. or A. F.) which connects the 
lateral dorsal medullary plates. A. L. Anterior end of the alimentary cavity. 
(Z. 2, A A ) 
Fig. 10 d. — T ransverse section through the posterior region of the head to 
show the reduction of the lumen of the neural canal. (Z. 2, A A.) 
Fig. 11. — Transverse section through the cervical region of an embryo of 
Amblystoma, somewhat more advanced than that of series d, showing the 
change of shape in the neural tube and canal. (Z. 2, A A.) 
PLATE XXVIII. 
Figs. 12 e and 13 e. — Longitudinal and nearly vertical sections of an em- 
bryo of Amblystoma (at a stage represented by Clarke’s Fig. 14). Fig. 12 e 
passes through the vertical plane at the hypophysis (Hp/i.) and the dorsal 
notochord ; Fig. 13 e at the anterior end of the notochord and the epiphysis 
(epL). (Z. 2, A A.) 
Figs. 14 f, 15 f, and 16 f. — H orizontal sections of an embryo of Ambly- 
stoma at the same stage as the preceding two figures. Fig. 14 f passes 
through the hind-brain and part of the dorsal medulla. Fig. 15 f passes 
through the mid-brain and the dorsal part of the alimentary cavity. Fig. 16 f 
passes through the hypophysis and the anterior part of the fore-brain. 
(Z. 2, A A.) 
Figs. 17 g and 18. — Longitudinal median vertical sections of two embryos 
of Amblystoma. 17 g is older than the stage of series e and f, and 18 is older 
than 17 g. These sections, together with 12 e, show the development of the 
lower jaw, the formation of the mouth, and the hypophysis. (Z. 2, A A.) 
Figs. 19 — 23 ine. — Longitudinal median vertical sections of successive 
stages of Frog-embryos, showing the formation of the fore-brain, the hypo- 
