xii Illustrations 



FIGURE 



11. Stages in the Early Development of the Frog, 



Seen Obliquely from the Posterior End. 

 (After Ziegler from Marshall) ... 30 



12. A Sagittal Section of Embryo Shown in Fig. 



n, D. B Sagittal Section of the Embryo 

 Shown in Fig. n v E (After Marshall) . 33 



13. Transverse Section of a Frog Embryo during 



the Formation of the Neural Canal. (After 

 Marshall) ....... 35 



14. Sagittal Section of a Tadpole at the Time of 



Hatching. (After Marshall) ... 39 



15. Transverse Section across the Middle of the 



Embryo Shown in Fig. n, D. (After Mar- 

 shall) 41 



16. The Brain of the Frog. (After Marshall) . 44 



17. Longitudinal Vertical Section through the 



Anterior End of a Tadpole Shortly after 

 Hatching. (After Marshall) ... 47 



1 8. Longitudinal Vertical Section through the 



Anterior Part of a Tadpole about the Time 

 of Appearance of the Hind Legs. (After 

 Marshall) ....... 49 



19. Half Sections in the Transverse Plane of a 



Tadpole 10 mm. Long (left half) and of a 

 Tadpole 12 mm. Long (right half). (After 

 Marshall) 52 



20. Transverse Section through the Head of a 



Tadpole 6^4 mm. in Length, about the 

 Time of Hatching. (After Marshall) . 54 



