3o8 SCOTT. . [Vol. I. 



PLATE X. 



Fig. 22. Sagittal section in a lateral plane through the nasal capsule of 

 adult Petromyzon. 



Figs. 23 and 24. Horizontal sections through the anterior portion of the 

 head of two embryos just before hatching. 



Fig. 25. Ditto, larva of 4.8 mm. This section is very oblique and almost 

 between the horizontal and transverse planes (after Balfour). 



Fig. 26. Ditto ; larva of 8 mm. 



Fig. 27. Sagittal section in a lateral plane through the eye of a larva of 

 22 mm. 



Fig. 28. Transverse section through the hind-brain of an embryo of the 

 15th day, showing the auditory involution. 



Fig. 29. Transverse section through the head of a ripe embryo. 



Figs. 30-33. Sections through the auditory vesicle of Ammoccetes : Fig. 

 30, transverse, larva of 8 mm. ; Fig. 31, larva of 14 mm. ; Fig. 33, larva of 23 

 mm., showing the opening of the commissure into the vestibule ; Fig. 33, 

 horizontal, larva of 23 mm. 



Fig. 34. Transverse section of the posterior part of the head, larva of 

 53 mm. 



Fig. 35. Horizontal section through the olfactory organ and nerve of 

 small Ammocoete (32 mm.). 



