50 McCLURE. [Vol. IV. 



The olfactory neuromere (Figs. 7, 8^, 9 ; Nm /.) is connected 

 with the olfactory nerves, which arise from the neural crest, 

 according to Marshall, in exactly the same manner as the sen- 

 sory roots of segmental nerves. He also states that the olfac- 

 tory nerves arise before the cerebral hemispheres, and in the 

 Dog-fish, Trout, Salmon, Axolotl, Frog, Lizard, Turtle, and 

 Chick their development is fundamentally the same. 



Orr states that in the Lizard the olfactory nerves spring 

 laterally from the anterior dorsal (nasal) tip of the primary fore- 

 brain, and run a very short distance direct to the nasal thicken- 

 ings of the epiblast, in which they end. In the Chick it is 

 fundamentally the same. In addition to confirming Orr's state- 

 ment in regard to the origin and course of the olfactory nerve 

 in the Lizard, I find an exact correspondence in the Newt 

 (Figs. 7^r, "jb, "jc). Thus it is seen that in the Lizard, Newt, and 

 Chick the olfactory neuromere (anterior dorsal tip of primary 

 fore-brain) gives off (on each side) a mass of ganglion cells 

 which constitute the roots of the olfactory nerves. This mode 

 of origin, as we have already seen, is exactly the same as that 

 described for the sensory roots in the segmental nerves of the 

 spinal cord and hind-brain. Therefore I think it is safe to say 

 that the olfactory nerve is the sensory division of the segmental 

 nerve which belonged to the olfactory neuromere, which accords 

 with Marshall and Beard, who upon entirely different grounds 

 consider this a true segmental nerve. 



General Summary. 



It has been my endeavor in the preceding pages to show that 

 a continuous and symmetrical series of folds (neuromeres), in- 

 creasing in size anteriorly, extend from the lateral walls of the 

 embryonic brain, throughout the entire length of the neuron, 

 and that these neuromeres are the remains of the primitive 

 segmentation of the neural tube. 



1. By proving that a conformity exists in the structure of 

 these neuromeres throughout the entire length of the neuron. 

 (See typical structure of neuromeres.) 



2. That all of the neuromeres in the spinal cord, four in the 

 hind-brain, and one in the primitive fore-brain, give rise to 

 dorsal or sensory roots. 



