No. 2.] EMBRYOLOGY OF THE LIZARD. 335 



appeared in the lateral walls of the hind-brain and the region 

 of the thalamenccphalon a number of symmetrical construc- 

 tions, giving the walls in horizontal, longitudinal, sectional an 

 undulated appearance. Kupjfer^ has given a short account, 

 without figures, of a similar condition in embryos of osseous 

 fishes, but did not discover the relation of these parts to the 

 cranial nerves. Kiipjfer has called these parts " Medullar-fal- 

 ten ; " but as the word " medullary folds " has another English 

 meaning, I have adopted for them the word " neuromeres." The 

 general appearance of the neuromeres is shown in the sections 

 of the hind-brain in series C. Each neuromere is separated 

 from its neighbors by an external dorso-ventral constriction, 

 and opposite this an internal sharp dorso-ventral ridge, — so 

 that each neuromere (/.c, one lateral half of each) appears as 

 a small arc of a circle. The constrictions are exactly opposite 

 on each side of the brain. Fig. 6, PI. XII., shows the arrange- 

 ment of the cells in the neuromeres at a very early stage. The 

 elongated cells are placed radially to the inner curved surface of 

 the neuromere. The nuclei are generally nearer the outer sur- 

 face, and approach the inner surface only toward the apex of the 

 ridge. On the line between the apex of the internal ridge and 

 the pit of the external depression, the cells of adjoining neuro- 

 meres are crowded together, though the cells of one neuromere 

 do not extend into another neuromere. This definition of ad- 

 jacent neuromeres presents, in some sections, the appearance 

 of a septum extcndiiig from the pit of the external depression 

 to the summit of the internal ridge {Spt). This septum may 

 be nothing else than those parts of cell-walls which form the 

 boundary line of the neuromeres, and which are made con- 

 spicuous by lying in a straight line. Of the neuromeres in the 

 hind-brain, five are distinctly marked with the above character- 

 istics, and are of equal extent in length. The most anterior of 

 these gives off (on each side) from its dorsal half a mass of 

 ganglion cells, constituting the root of the fifth nerve. The 

 second gives ofi* ventrally, and at a much later period, the sixth 

 nerve. The third neuromere gives off, in a manner similar to 

 the first, the single ganglionic root of the seventh and eighth 

 nerves. The fourth neuromere gives off no nerve, but the 



' Kupffer. — Primare Metamerie des Neuralrohrs der Vertebraten, Sitzing d. 

 Math', — physische Classe vora 5 Dec, 1885, Mlinchen. 



