ILLUSTRATIONS 



The Development of the Frog 



FIGURE PAGE 



1. Early Stages in the Development of the Frog 



Embryo. (From a photograph of Ziegler's 

 wax models) . . . Frontispiece 



2. Egg of Starfish. (After Gegenbaur) . . 6 



3. Ovarian Egg of Frog ..... 7 



4. Various Stages in the Development of the 



Frog. (After Brehm from Marshall) . 8 



5. Vertical Sections of Segmenting Eggs . . 19 



6. Sections of Three Stages of Segmentation of 



the Frog's Egg 20 



7. Longitudinal Vertical Section of a Frog Em- 



bryo, Showing Commencing Invagination. 

 (After Marshall) 22 



8. Longitudinal Vertical Section of a Frog Em- 



bryo at a Later Stage in the Formation of 



the Mesenteron. (After Marshall) . . 23 



9. Longitudinal Vertical Section through a Frog 



Embryo, Showing the Completion of the 

 Mesenteron. (After Marshall) ... 26 

 10. Transverse Section through the Middle of a 

 Frog Embryo, at about the Stage Repre- 

 sented in Fig. 9. (After Marshall) . . 27 



