10 CONARIO-HYPOPHYSIAL TRACT. 



Pig. 4. 



1 



Mammalian embryo. — 1. Neural axis. 2, Vertebral axis. 3. Cere- 

 bellar vesicle and medulla oblongata = epencephalon. 4. Optic vesicle 

 mesencephalon. 5. Thalamencephalon, or vesicle of third ventricle. 

 6. Cerebral vesicle = prosencephalon. 7. Pineal portion, 8. Pituitary 

 or infundibular portion, of the conario-hypophysial tract. 9-10. Bucco- 

 branchial cavity. 11. Stomach. 12. Small intestine. 13. Large in- 

 testine. 14. Yent. 15. Heart and chief blood-vessels. 16. Primordial 

 kidney. 17. P'rinary bladder. 18. Pedicle of allantois, 19. Pedicle of 

 umbilical vesicle. 20. Liver. 



convergence of its side-walls, it is converted by their con- 

 fluence into a tube. 



Passing over the histological steps in the- formation of the 

 grey and white matters — " columnar " being converted into 

 " multipolar " cells — and the reduction thereby of the canal to 

 the minute central one of the adalt myelon, what here con- 

 cerns ray argument is the progressive forward extension of 

 the cord, with corresponding expansions into the beginnings 

 of the " hind brain," or epencephalon (fig. 4, a), of the mid 

 brain or mesencephalon (id. 4), and of a large vesicle, " thala- 

 mencephalon " {i5. b), dividing the latter from the fore brain, 

 or " prosencephalon " (id. e). All these expansions, as shown 



