456 J. GKAllAM KEKB. 



c. Stage 31, tlic gland at, its full developiiieut. 



u. Stage 35, illustrating the atrophy of the gland, b.v. Blood-vessel ; 

 /. Fat globules; ph. Phagocytes. 



Fig. 4. — Portions of skin stripped off two Lepidosirens of Stage 38 

 (belonging lo the same brood). Zeiss A, oc. 4. 



A. Killed at 2 p.m. in daylight. 



B. Killed at 9 p.m. in faint lamp-light, 



c, Cliromatophores of Type A; c.^ Cliromatophures of Type B; 

 g.c. Unicellular gland ; gl. Multicellular gland. 



Fig. 5. — Vertical sections through the skin of young Lcpidosirens 

 killed under different conditions of light and darkness. Zeiss D, oc. 2. 



A. Prom white enamelled vessel in bright daylight. 



B. From deep shade under a verandah. 



c. Dropped into the fixing agent at 9 p.m. in darkness. 

 [Note. — In a the epidermis has become pushed upwards by the action 

 of the acetic acid in the fixing solution.] 



Fig. 6. — Illustrating the formation of the lining of the buccal cavity. 



A. Part of a sagittal section of a Lepidosiren of Stage 30. Zeiss D, 



oc. 2. 



B. Part of a similar section of an embryo of Amblystonia, measuring 



7' 5 mm. in length. 

 Luc. Still solid, yolk-la'len cell-mass in position where the 

 buccal cavity will develop later; e^. Superficial layer of 

 ectoderm ; e^. Deep layer of ectoderm ; stom. Lining epi- 

 thelium of buccal cavity arising in situ. 



PLATE 26. 



Fig. 7.— Illustrating the development of the teeth. 



A. Portion of a section parallel to the sagittal plane at Stage 31, 



showing one of the palato-pterygoid tooth germs. The buccal 

 cavity [b.c.) is beginning to form, but it is still closed anteriorly 

 (to the right in the figure). Zeiss D, oc. 2. 



B. Portion of a similar section at Stage 35. Zeiss D, oc. 2. The figure 



shows the enamel cap (<?.) and the commencing formation of 

 dentine {d.). e.o. Enamel organ; o. Odontoblast. (The split 

 round the enamel is, as shown by other sections, artificial.) 



