3Q5 



Mc Cluee, followiug Orr's work, demonstrated the neuromeric 

 segmentation of the neural tube throughout its whole extent, and 

 published a preliminary announcement of the same in 1889. He 

 showed the presence in the spinal cord of segments continuous with 

 those in the brain and histologically similar to them. He examined 

 these structures in embryos of the chicken, Amblystoma and the 

 lizard (Anolis). He fixed upon six, in the chicken and lizard, and 

 five, in Amblystoma, as the number in the hind-brain of each respec- 

 tively. He found two in the fore-brain but left the number in the 

 mid-brain undetermined, expressing the view, however, that there are 

 two neuromeres in that brain region. 



Miss Platt's work (1889), on primitive axial segmentation of the 

 chick, agrees, in so far as neuromeric segmentation is concerned, with 

 that of her predecessors, except as regards the relation of the nerve 

 fibres to their corresponding neuromeres. According to her observations 

 they spring primarily from the concavity between two neuromeres and 

 not from the outward crest of a neuromere. 



1 shall show, later in this article, how my observations, on the 

 place of origin of the fifth nerve, agree with those of Miss Platt. 



Waters studied especially the mid-brain of Teleosts. He con- 

 firmed and extended the observations of Orr and Mc Clure. He 

 counted eleven neuromeres in the entire brain region: six in the 

 hind-brain, two in the mid-brain and three in the fore-brain. He did 

 not find neuromeres in the brain of the cod earlier than the sixth 

 day. Waters complete paper published in June 1892 is the most 

 recent piece of literature bearing on the subjeet I have seen. 



Besides the neuromeric segmentation there is a metamerism ex- 

 pressed in the mesoderm of the head, and upon that anatomical con- 

 dition valuable work has been done by van Wijhe, Balfour, 

 Marshall, Dohrn, Killian, Oppel and others. In the present 

 article, I wish to waive a consideration of the mesoblast of the head, 

 and confine attention to segmentation in the epiblast, since a great 

 deal of what I have to say on that part of the subject is new, and 

 is, I believe, now made known for the first time. The mesoblastic 

 segmentation of the head of Torpedo ocellata has been beautifully 

 worked out by Dohrn and Killian, but in Squalus acanthias it re- 

 mains to be studied. 



It appears, then, that our knowledge regarding the segmentation 

 in the neural tube has passed through the phase of simple observation 

 of its occurrence (von Baer '28, Remak '50, Dursy '69 and others) 

 and has grown by successive additions, to the recent conception of 



27* 



