Saroent. — Optic Reflex Apparatus 



PLATE 1. 



Fig. 1. Petromyzon planeri, 26th day. Parasagittal section (anterior at right, a 

 httle to the right of median plane) of tlie anterior portion of the roof 

 of the mesencephalon, showing the optic reflex cells with their axons 

 protruding into the ventricle. X SOO ca. 



Fig. 2. Petromyzon planeri, 30th day. Parasagittal section (anterior at right, a 

 little to one side of the median plane) of the anterior portion of the 

 mesencephalic roof, showing the optic reflex cells with tlieir axons pass- 

 ing into tlie mesocoele. X 800 ca. 



Fig. 3. Petromyzon planeri, 30th day. Transverse section, a little caudad of 

 the posterior commissure, showing the median nidulus of optic reflex 

 cells. X 900 ca. 



Fig. 4. Petromyzon planeri, 30th day. Transverse section through the dienceph- 

 alon and the ganglia habenulae. Viewed from in front. X 205. 



Fig. 5. Petromyzon marinus, adult, 30 cm. Parasagittal section through the roof 

 of diencephalon and base of right ganglion habenula, showing the 

 liabenular elements of Reissner's fibre entering the diacoele. X 700 ca. 



Fig. 6. Petromyzon marinus, adult. Transverse section through the right lateral 

 ventricular groove, and horn, showing the axons of the optic reflex cells 

 passing through the ependymal thickening, entering the ventricle and 

 uniting in fasciculi. X 700 ca. 



Fig. 7. Petromyzon marinus, adult, 30 cm. Transverse section through the ante- 

 rior portion of the mesencephalon and posterior commissure, showing 

 the lateral ventricular grooves with tlieir ependymal thickening, and 

 a few of the large optic reflex cells. X 35. 



Fig. 8. Petromyzon marinus, adult. Sagittal section of the posterior portion of 

 the spinal cord througli the ventriculus terminalis, within which is the 

 contracted and convoluted Reissner's fibre. X 385. 



