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The investigations of () r r and Me C 1 u r e are not only the most recent 

 but the most definite and exhaustive and have well established the fol- 

 lowing points': 



1 . The hind-brain is made up of five or six symmetrical constric- 

 tions (neuromere s) from the crests of which the cranial nerves 

 of the corresponding region take origin, with the exception of the 

 VIII. nerve. 



2. The fore -brain contains neuromeres corresponding in every 

 characteristic of structure to those of the hind-brain, the number being 

 doubtful. 



3. The spinal cord also consists of neuromeres similar to those 

 of the medulla from which the sensory roots of the spinal nerves ori- 

 ginate. 



4. All these segmentations whether giving origin to nerves or not 

 degenerate rapidly and early. 



It will be seen that these investigations left the primitive condition 

 of the fore-brain very doubtful and that of the mid-brain even more 

 so. I have attempted by the study of fish and amphibian embryos to 

 establish the points already made and to add some new ones, regarding 

 especially the fore- and mid-brain. The forms used were Gadus morrhua 

 and Ambty stoma punctatum embryos. The following is a brief review 

 of Part I. 



I have found that in general the neuromeric segmentation appears 

 at a relatively late period in the Cod. Sections of early stages (6 to 9 

 days) show the neuron as a perfectly straight and narrow tube , the 

 walls of which are in apposition along the median line but not fused 

 and in which there is no proper lumen anterior to the 4 th ventricle. 

 A few of the sections (9 days) show some of the characteristic hind- 

 brain segmentations but none appear in the other regions. 



In embryos of about 10 days incubation, cranial flexure is quite 

 marked and individual sections of the entire brain area are not ob- 

 tainable. However the ventral portion still preserves its simple tubu- 

 lar character, the walls being narrow and uniform in thickness , but 

 enclosing between them a considerable lumen. In embryos of this age 

 the neuromeres of the anterior regions first appear. 



Fore- brain. 



Ibis area has a peculiar club-shaped or trefoil appearance. Directly 

 in front of the eyes and almost in apposition with the forward end of the 

 primitive fore-brain lie the olfactory vesicles. These I find are connec- 

 ted with the brain by a short thick mass of cells on either side, the 

 I. pair of nerves, which even in late stages have no connection with 



