No. 3-] THE VERTEBRATE HEAD. 525 



them from the side, and comparing them with earlier stages, 

 that we shall be able to identify the boundaries of the original 

 segments. 



Just what is taking place during the appearance of the crests 

 is not now clear to me. I have heretofore assumed that it 

 signified a union of the original segments, the anterior half of 

 one with the posterior half of the segment lying just in front of 

 it, but at present I am inclined to question that interpretation. 

 The crests on the upper margin are between two neuromeres 

 as designated by Orr, and they correspond in position to his 

 inner ridge. The point of origin (motor fibres) of the fifth, 

 seventh, and eighth nerves (so far as it may be determined by 

 surface view) is now clear ; they arise, as Miss Piatt has 

 described them in the chicken, from the concavity (on the 

 lower margin) between two neuromeres. This will receive 

 fuller consideration under the heading. The Nerves. It will 

 be interesting to note incidentally, in this figure, the very 

 large development of mid-brain over that in Figs. 33 and 34, 

 and the consequent crowding forwards of the first accessory 

 optic vesicle. The latter structure is also much reduced in size, 

 and with its fellow is in the region of the thalamencephalon. 



None of these figures have shown the condition of the 

 entire neural tube. The segmentation so clearly seen in Figs. 

 34, 35, and 36, represents only a part of the actual segmenta- 

 tion, viz.y that in the uppermost part of the neural tube. The 

 rest of the tube is too much covered by mesoblast to be seen 

 without dissection. The upper part of the tube has — in the 

 region of the hind-brain — two thickened lateral bands of 

 cells, which form a border on either side of the neural tube ; 

 these are the segmented parts that are visible from surface 

 observation. I have found it necessary to remove the over- 

 lying layer of mesoblast and the outer epidermic stratum, and 

 completely expose the walls of the brain. When thus laid 

 bare, the walls of the neural tube show in a most satisfactory 

 manner. The ten or twelve figures following those just 

 described show dissections of this kind. 



PI. XXVIII, Fig. 41, shows the surface view of an embryo 

 slightly older than that represented in Fig. 33, and Fig. 42 



