PLATE I. 



Fig. I. Transverse section of ovarian egg of Thyridopteryx. OE Ovarian 

 epithelium. YS Yolk spherules. P Protoplasmic stratum surrounding yolk. 

 CH Chorion. 



Fig. I'. Portion of same section more highly magnified. 



Fig. II. Segmenting egg of Thyridopteryx. YC Yolk cells most of which 

 iiltimately reach the surface. 



Fig. III. Transverse section of egg of Thyridopteryx with blastoderm 

 partially formed about yolk. Yolk spherules are not represented. 



Fig. IIP. Yolk cells at this stage reaching the surface of the egg. 



Fig. IV. Transverse section of egg of Thyridopteryx showing completed 

 blastoderm (BL) ; early embryo (E), and commencing amniotic folds (AMF). 



Fig. IV. Highly magnified embryonic area of same section. 



Fig. V. Transverse section of egg of Thyridopteryx showing embryo, (E) 

 in cross section situated in centre of yolk. Amnion covering its ventral surface is 

 not represented. The yolk is segmented. 



Fig. VI. , Hexagonal blastoderm cells. 



Fig. VII. Transverse section of embryo of Thyridopteryx showing formation 

 of inner layer (I). 



Fig. VIII. Horizontal section of embryo of Thyridopteryx showing proce- 

 phalic lobes (PC) at anterior end. 



Fig. VIII'. Longitudinal section of embryo at same stage showing segmen- 

 tation of inner layer (IS). 



Fig. IX. Horizontal section of embryo at same stage as that represented in 

 Fig. VIII showing its relations to blastoderm BL. 



Fig. X. Transverse section of Thyridopteryx embryo showing separation of 

 inner layer (L) beneath the blastopore BH. 



Fig. X'. Same section less highly magnified showing its relations to blasto- 

 derm (BL). 



Fig. XI. Transverse section of older embryo showing origin of nervous 

 system (NS). 



Fig. XII. Nervous system separated from outer ectoderm. Elongated cells 

 at bottom of median groove. 



Fig. XIII. Transverse sections through region of procephalic lobes. 



Fig. XIV. Transverse section through same region of older embryo. Inner 

 cells of lateral portions specialized as nerve cells. 



