OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 81 



yolk attached to the under surface of tlie yolk-sac. R. M. Roof of mid-brain. 

 iS". ^. Shell-breaker (patch of cornified epidermis on snout). S. C. Segmenta- 

 tion cavity (subgerminal cavity). S. E. M. Sub-embryonal membrane. S. En. 

 Serous envelope. Si. "Sickle," = mass of mesoblast growing out from the 

 primitive streak behind. S. M. Superior maxillary process. Som. P. Somato- 

 pleure. S^i.C. Spinal cord. Spl.P. Splanchuopleure. S.T. Sinus terniinaiis. 

 Sta. Stalk with vitelline vessels attaching embryo to yolk-sac. Slom. Stonio- 

 dseum. aS". F. Segmental vesicle. T. Tail. T^. Thyroid gland. Thal.l:\\&\d.- 

 mencephalon. Tr.L. Transverse line along which the anterior end of the 

 embryo is attached to the overlying blastoderm, and where the yolk-sac turns 

 forwards to form the outer layer of the pro-amnion. V. A. Vitelline artery. 

 1 V.A. First visceral arch. 2 V.A. Second visceral arch. 3 V.A. Third 

 visceral arch. 4 V. A. Fourth visceral arch. 5 V. A. Fifth visceral arch. 

 1 V.C. First visceral cleft (hyomandibular). 2 V.C . Second visceral cleft. 

 3 V. C. Third visceral cleft. 4 V. C. Fourth visceral cleft. V. V. Vitelline 

 veins. V. 4. Fourth ventricle, W. B. Wolffian duct. W. R. Wolffian ridge. 

 IF. T. Wolffian tubule. X. (Fig. 47). Junction of clear and yolk-laden areas 

 of yolk-sac. X. (Fig. 83). Spot where serous envelope is still united with true 

 amnion. Y. Yolk. Y.S. Yolk-sac. 



Note. — The microscopic drawings have in almost all cases been made with 

 the aid of Abbe's camera lucida. 



Figs. 1—5. Stage C. 



Fig. 1. — Embryo 9, upper surface, with portion of surrounding blastoderm ; 

 drawn from spirit specimen as an opaque object, x 10. 



Fig. 2. — Embryo 9, lower surface, with the sub-embryonal membrane partly 

 torn away ; drawn as before. X 10. 



Fig. 3. — Embryo 9, seen from above as a transparent object after staining 

 with borax carmine and clearing in oil of cloves. Zeiss A (with the bottom 

 lens removed), ocular 1. 



Fig. 4. — Embryo 9, longitudinal vertical section, passing through the head- 

 fold in front and the blastopore behind. The dotted line indicates the posi- 

 tion of the sub-embryonal membrane. Zeiss A, ocular 1. 



Fig. 5. — Embryo 5 (slightly younger than 9, before the formation of the 

 blastopore), seen from above as a transparent object in Canada balsam, after 

 staining with borax carmine. Zeiss A (with the bottom lens removed), 

 ocular 1. 



Figs. 6—14. Stage D. 



Fig. 6. — Embryo 58, upper surface, with portion of surrounding blasto- 

 derm ; drawn from spirit specimen as an opaque object, x 10. 



Fig. 7. — Embryo 58, seen from above as a transparent object after staining 

 with borax carmine and clearing in oil of cloves. Zeiss A (with the bottom 

 lens removed), ocular 1. 



VOL. 42, PAKT 1. — NEW SEE. F 



