182 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Plate 7, Fig. 47, corns, p.). This constrictiou corresponds, instead, with 

 a point just behind the epiphysis, and is separated from the posterior 

 commissure by that portion of the brain which KupfFer has named 

 Schalthirn, or diencephalon (the Schaltstiick of Burckhardt). Neither 

 of the so called neuromeres (Orr) is in relation with a nerve, motor, or 

 sensor, and neither possesses a dorsal expansion of its own. 



A parasagittal section of the next older stage represented is seen in 

 Figure 47, Plate 7 (compare Fig. 19, Plate 4); it is of an embryo with 

 65 somites (Balfour's stage K), and the changes in the anterior brain 

 region are seen to be considerable. In the dorsal portion of the region 

 called primary forebrain, i. e. the region anterior to the constriction in 

 which the "thalamic nerve" (thi, Fig. 18, Plate 4) lies, two expansions 

 now appear. These are median, unpaired, and separated from each 

 other by a constriction which extends toward, but does not reach, the 

 optic stalk. The anterior expansion is the prosencephalon (Grosshim, 

 epencephalon of Kupflfer), which involves, as determined by His (88''), 

 the " Deckplatte " and both " Flilgelplatten." The second, which at this 

 stage is a simple expansion, later becomes differentiated into " Zirbel- 

 polster " (KupfFer's parencephalon, Nebenhirn) and the epiphysis. The 

 latter, according to His, is derived from the " Deckplatte " only. The 

 primary constriction between forebrain and midbrain is marked in Fig- 

 ure 47 by the dorso-ventral line, behind the second expansion. The 

 midbrain now shows three lateral expansions. The anterior is bounded 

 in front by the primary constriction between forebrain and midbrain, and 

 behind, as in the previous stage, by the ventral (and now lateral) con- 

 striction which extends dorsad toward the posterior commissure from a 

 point just in front of the chief root of the oculomotor nerve. The sec- 

 ond dilatation has as its posterior boundary a ventral constriction which 

 I do not consider of morphological importance, because it simply cor- 

 responds with a point of flexure of the ventral wall of the tube, never 

 extends to a dorsal position, and has no corresponding inner ridge. The 

 constriction exists, however, at this stage, and forms the posterior boun- 

 dary of a neural segment related to the oculomotor nerve. Behiud this 

 lies a third expansion, faintly marked anteriorly and also posteriorly, 

 where it merges into the isthmus. In later stages the trochlear nerve 

 arises from the region of the posterior constriction of this expansion ; it 

 is the chiasma of fibres of this nerve which defines the posterior con- 

 striction of the midbrain vesicle. In this stage, as in the preceding, the 

 midbrain vesicle remains dorsally a simple expansion, the constrictions 

 affecting only its lateral and ventral walls. 



