Plate X. 



Arabic numerals placed between every two sections indicate the number 



of sections that intervene between those two (exclusive). 



Certain marks on the left of each section are for the identification of the 



sections and do not concern the reader. 



Series XIII. a~l. — Sections through an embryo of Chelnnia caouana, with about 18- 

 19 mesoblastic somites, taken out 11^ days after deposition ; in general 

 appearance very much Hke the Clemmys embryo given in Fig. 15. For 

 detailed description see p. 65. C'Cx 2. (B 13 p ) 



Series XIII. m. — The fourth section behind / in the above series, shows the remnant 

 of the yolk-plug. For detailed description see p. 67. DDx2. 

 Amm. crtr.=-Amniotic cavity. 

 Coe/'^^ Extra-embryonic body-cavity. 

 Cotifj. ??irt.s-s^Coagulated mass. 

 i?7>/W.=Epiblast. 

 M<?.soi/.=Mesoblast. 



Seio-Amn. CV)»».=Sero-amniotic connection. 

 Diagrams I-IX. — 



Diag. I-IV show the course of development in Elasmobranchii. 

 Diag. V-IX show the course of development in Chelonia. 

 For detailed description see p. 101 et seq. 



Ym. l=Primary yolk-mass. 

 Ym. 2=Secondary yolk-mass. 

 Diag. IVr/ shows the tail part in IV on a larger scale. 

 Dias. l\.a „ „ IX „ ,, 



