PLATE II. 



Fig. XV. Transverse section of advanced Thyridopteryx embryo. Nervous 

 system (NS) separated from the ectoderm. Amnion (AM) growing dorsally. Por- 

 tion of the inner layer (IE) becoming specialized as endoderm. 



Figs. XVI-XXI. Longitudinal sections of Thyridopteryx embryo. 



Fig. XVII. Is nearly median. Numbers 1-17 indicate somites which have- 

 ganglia corresponding to them. 



Fig. XXII. Cephalic portion of XVII highly magnified. 



Fig. XXIII. Last two thoracic (Nos. VI, VII) and first two abdominal ganglia 

 (Nos. VIII, IX) of figure XVIII, highly magnified. 



Fig. XXIV. Transverse section of advanced Thyridopteryx embryo showing 

 approximation of amnion folds (AM) on dorsal side of embryo. Endodermic por- 

 tion of inner layer (IE) beginning to shut yolk into body cavity. Migratory meso- 

 derm (I') growing round yolk before the endoderm. 



Figs. XXV, XXVI. Transverse sections of same embryo, to show the approxi- 

 mation of amniotic folds on dorsal side of embryo. 



Figs. XXVII-XXVIII. Transverse sections of the head of an advanced 

 Thyridopteryx embryo. The plane of the section is approximately an imaginary 

 line joining numbers i and 2 in Fig XVII. 



Fig. XXIX. A diagramatic representation of the under surface of the head 

 of an advanced embryo. 



Fig. XXX 1 . A diagramatic lateral view of the head of an embryo at a corre- 

 sponding stage of development. 



