NEAL : NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 169 



being exactly frontal on account of the torsion of the embryo. The lat- 

 eral walls of the neural tube are seen in the figure to be thickened in 

 that region -which lies just posterior to the constriction opposite van 

 Wijhe's 3d somite. A comparison of many frontal and sagittal sections 

 leaves no doubt that this thickening lies in the region of neuromere IV. 

 That expansion of the neural tube which lies between the 2d and 3d 

 somites, and which is separated by an external constriction from neuro- 

 mere IV behind and from the midbrain vesicle (encephalomere II of my 

 figures) in front, is the most anterior of the primary expansions or 

 encephalomeres of the hindbraiu. It has been called by Zimmermaun 

 ('91) " Hinterhirn." This corresponds to the third expansion of the 

 neural tube in the chick (Fig. 44, Plate 7), as may be determined by its 

 relation to the acustico-facialis Anlage and the auditory invagination. 

 Failure correctly to identify this vesicle in the chick led Miss Piatt ('89) 

 to call the second vesicle, viz. the primary midbrain, the Idndhrain. 



At a later stage, when 17 to 18 somites are differentiated, a well 

 marked local thickening in the posterior half of encephalomei'e III 

 appears.-"- A frontal section of an embryo at this stage, showing neuro- 

 mere IV as a local thickening posterior to neuromere III, is seen in 

 Figure 23, Plate 5. Encephalomere III is separated by a constriction 

 from encephalomere II. At this stage, then, only four of the hindbrain 

 neuromeres (III, IV, V, and VI) are differentiated, and the conditions 

 remain the same when one more somite is formed. 



In a similar frontal section of an embryo with 19 somites, such as is 

 represented in Figure 24, four symmetrical thickenings of the lateral 

 walls of the hindbrain (III-VI) appear. Opposite neuromere V lie 

 the cells of the Anlage of the acustico-facialis nerve (blue), and opposite 

 neuromere VI the thickened auditory epithelium. Xeuromere VII is 

 not present at this stage, and it does not begin to be differentiated until 

 after one or two more mesodermal somites are formed, when a faintly 

 marked dorsal and ventral dilatation appears in the region of the neu- 

 ral tube just behind neuromere VI (Fig. 9, Plate 3). The lateral walls 

 of this neuromere never become so markedly thickened as the walls of 

 the other neuromeres, nor does the neuromere show a constriction at its 

 posterior border before the embryo reaches the condition of Balfour's 

 stage H, and then only a faintly discernible one. A cross section 



1 Such a secondary subdivision of encephalomere III. (" Hinterhirn ") occurs in 

 the chick as in S. acanthias. I regard the primary vesicle as of different morpho- 

 logical value from that of its subdivisions, for reasons which will be made more 

 apparent when the relations of the vesicles are studied. 



