284 



THE PROTOCHORDATA. 



evagination from the floor of the brain, occurring behind 

 the anterior terminal extremity of the brain. 



It follows that the morphological anterior extremity of 

 the craniate brain coincides with the median lobiis olfac- 

 torius impar, which also represents the point of last con- 

 nexion of the medullary tube with the superjacent ecto- 

 derm. The lobus olfactorius impar lies in the anterior 

 vertical wall, which forms the boundary of the primary 

 fore-brain in front, known as the lamina terminahs. Rabl- 

 RtJCKHARD has also observed the median olfactory lobe in 



0:C -irif 



Fig. 134. — Sagittal section through the head of an embryo of Acanthias. 

 (After Rabl-Ruckhard.) 



ax. Position of anterior commissure, al. Alimentary canal, cer. Cerebellum. 

 ch, Notochord ; the black shading below the notochord indicates the aorta. 

 /*. Fore-brain, h.i. Hind-brain, hy. Hypophysis, already shut off from the 

 stomodoeum and lying as a closed sac at the base of inf, the infundibulum. 

 1,0. Lobus olfactorius. m. Mouth, m.b. Mid-brain, o.c. Optic chiasma. p.b. 

 Pineal body (epiphysis). 



the Selachian embryo (Fig. 134), and it has since been 

 found by Burckhardt in other forms. 



It can thus hardly be doubted that the median rudi- 

 mentary olfactory lobe of the embryos of the higher 

 Vertebrates is homologous with the lobus olfactorius of 

 Amphioxus (Fig. 51), and, like the latter, represents the 

 remains of the neuropore. In Amphioxus, however, the 



