442 W. H. GASKELL. 



nerve, op. th. Optic tlialauius. o. w. Anterior wall of optic vesicle, piy 

 Dorsal pineal eye. />«2- Ventral pineal eye. p. com. Post-commissure. U, 

 Retina of pineal eye (layer of nerve-end cells), r. Layer of rhabdites. ra. 

 Raphe, r. aq. Raphe of aqueduct, st. Lining epithelium of central cavity 

 of brain (epithelium of stomach wall), tr. Trabeculse. t. th. taenia thalami. 

 V. Vitreous layer or corneagen. V. aq. Ventricle of aqueduct. V. c. Ven- 

 triculus communis. V. lat. Lateral ventricle. F3. Third ventricle. F,. 

 Fourth ventricle. 



Fig. L — Dorsal view of brain of Ammocoetes. 



Fig. 2. — Transverse section through the infundibular region of the brain 

 of an Ammocoetes, immediately after its metamorphosis. Picro-carmine and 

 eosine preparation. The section cuts through the very commencement of 

 the right ganglion habenulse. 



Fig. 3. — From the same series as Fig. 2. The section cuts through the 

 posterior commissure and the very commencement of the notochord. Figs. 2 

 and 3 correspond closely to Ahlborn's Figs. 27 and 26 respectively. 



Fig. 4. — Transverse section through the commencement of the formation 

 of the fourth ventricle. Hsematoxylin preparation. 



Fig. 5. — Magnified drawing of the ventral portion of one side of the nerve- 

 substance of the cord in Fig. 4. 



Pi(j, 6. — Horizontal section through the brain of a full-grown Ammocoetes. 

 Osmic preparation. 



Pig. 7. — Magnified drawing of the part of the brain marked x in Fig. G, 

 taken from a hsematoxylin preparation of the brain of a full-grown Ammocoetes. 



Fig. 8. — From the same series as Fig. 6, to show the optic thalamus, third 

 ventricle, ventricle of the aqueduct, and the fourth ventricle. 



Fig. 9. — From same series as Fig. 2 and Fig. 3, to show substantia centralis 

 gelatiuosa and arrangement of groups of nerve-cells in the epicliordal portions 

 of the brain. 



Fig. 10. — Magnified drawing of a portion of Fig. 9. 



Fig, 11. — Transverse section through the conns post-commissuralis of 

 nearly full-grown Ammocoetes. Osmic preparation. To show how the limits 

 of the lumen are marked out by fat-globules. 



Fig. 12. — From the same series as Fig. 11. Magnified drawing of the grey 

 matter of the epichordal part of the brain, to show the formation of the raphe 

 and the limiting line of the layer of fat-globules. 



Fig. 13 a, b, c, d, e. — Selections out of a series of transverse sections 

 through the region of the conns post-commissuralis, to show the presence of 

 an occluded ventral duct in tliat region. Osmic preparation of the head of a 

 half-grown Ammocoetes. 



