DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 757 



the fourth ventricle. A very thin roof continues from the cerebellum and covers the 

 medulla oblongata, whose triangular floor is becoming better defined and dorso-ventrally 

 deeper. Laterally the cerebellum (cb) does not break continuity with the double fold of 

 the mesencephalon (mb), of which, indeed, it appears to be merely a thickened posterior 

 portion or third fold (PL VI. figs. 5, 6). This fact has led many anatomists to deny to 

 this fold the name of cerebellum. MM. Philipeaux and Vulpian regard it as merely a 

 third lobe of the optic mass, and they, with many others who follow Weber (Anatornia 

 comparata nervi sympathici, Leipzig, 1817), regard the two swellings passing along each 

 side of the medulla oblongata, and composed of grey vesicular matter, as the cerebellum. 

 MM. Philipeaux and Vulpian emphasise the view that the cerebellum consists of the 

 two hollow lateral masses which flank that part of the optic mass usually called cerebellum 

 (No. 118, p. 171). More recently Miklucho-Maclay has urged a similar view regarding 

 the Elasmobranch brain, and he interprets the prominent anterior portions of the corpora 

 restiformia as the true cerebellum. Beneath the cerebellum the medulla continues 

 forward and merges in the basal region of the thalamencephalon, and even in this 

 early condition distinctly turns upward, the curvature becoming marked somewhat later 

 (PL XXIV. figs. 1, 2). Eyder states that in Alosa this upward bend is not indicated 

 (No. 141, p. 504), but it is probably a notable feature in most Teleosteans. Dohrn, for 

 instance, indicates it in Belone and the Lophobranchs, and the result of it is that the fore- 

 brain is brought down below the front termination of the medulla oblongata, and a false 

 cranial flexure is thus effected. The actual relations, in regard to position, of the brain- 

 vesicles to, say the notochord, are not altered, nor does the head-region externally 

 present a marked flexure downward (PL XIII. figs. 1, 5) ; yet a longitudinal section 

 through the brain shows, as in the section of Anarrhichas (PL XXIV. fig. 2), the 

 mesencephalon raised up, while the prosencephalic region bends down. If we follow 

 the course of the medullary canal, we find in front of the cerebellum a marked 

 ascent — the hollow optic lobes occupying a much higher plane than the thalamen- 

 cephalon and the hemispheres, and the result is that, without the remarkable shifting 

 forward of the parts of the brain seen in the chick, the prosencephalon comes to 

 lie on the ventral side of the medulla and cerebellum. This modified flexure, which is 

 not comparable to the true and extensive bending down of the prosencephalon, e.g., of 

 Elasmobranchs, has this simple explanation, that without any very obvious displacement 

 of the other parts of the brain, the floor of the myelencephalon and metencephalon are 

 flexed upward, and, as a consequence, the mesencephalon is raised, and the thalamen- 

 cephalon and hemispheres come to occupy a distinctly ventral position. This false 

 flexure persists even after the embryo has emerged from the ovum, but with the fuller 

 development of the oral region it is at a later post-larval stage corrected. 



The remarkable displacement just described does not change the external aspect of 

 the embryo. The mid-brain still occupies the summit of the head — its greater pro- 

 minence being due to the process of median upheaval from below, which brings the 

 lateral margins of the optic lobes into proximity inferiorly with the eyes. The optic 



