170 bulletin: museum of compaeative zoology. 



through neuromere IV, which serves to show how greatly thickened 

 the lateral wall is at this stage, is shown in Figure 32, Plate 5. The 

 dorsal wall of this neuromere is considerably thicker than that of the 

 neuromeres anterior and posterior to it, possibly because few cells are 

 proliferated from this neuromere to form the ganglionic Anlage or neu- 

 ral crest. 



I pass now to a description of the hindbrain neuromeres (encephalo- 

 meres) at a stage with 28 or 30 somites (Balfour's stage H). Since at 

 this stage the neuromeres are clearly differentiated, and the thinning and 

 expansion of the roof of the hindbrain have progressed very little, this 

 is a most favorable stage for the study of the structural and histological 

 peculiarities of the hindbrain neuromeres.^ Figure 13, Plate 3, repre- 

 sents a cleared specimen at this stage, and Figure 25, Plate 5, a frontal 

 section of the same. Opposite neuromere III (Fig. 25) lies part of the 

 trigeminus Anlage ; opposite neuromere V lie the cells of the acustico- 

 facialis Anlage ; and opposite neuromere TI lies the thickened auditory 

 epithehum, which is just beginning to invaginate. The acustico-facialis 

 Anlage always remains in relation with neuromere V, so that this serves 

 as an excellent starting point in counting the neuromeres. In order to 

 get a clear conception of the structure of the neuromeres, cross, frontal, 

 and Sagittal sections are necessary. The series represented in Figures 

 36-38, Plate 6, are frontal sections taken at different levels (a, /3, -y, 

 Fig. 40, Plate 6) in the medullary tube. Only the right wall of the 

 mrdullary tube in the region of neuromeres IV and V is shown in de- 

 tail. The first section (Fig. 36) is dorsal, in the region of the " Deck- 

 platte." In this section it is seen that what Orr ('87) has said for the 

 Lizard (see page 167) is true for Squalus. The section reproduced 

 in Figure 37, more ventral than Figure 36, shows that the conditions 

 which obtain in the region of the lateral zones are somewhat different 

 from those of the dorsal zone. Since no sharp internal ridge exists, each 

 lateral half of a neuromere does not appear in section as an arc of a 

 circle, but as a thickening of the wall of the medullary tube. The cells 

 and nuclei are fewer in number and more crowded in the region of con- 

 striction between neuromeres. Although there is no inner concavity at 

 this level, the cells and nuclei (Fig. 37) show a radial arrangement sim- 

 ilar to that shown in Figure 36. The ventral section (Fig. 38) differs 

 in no essential respect from the dorsal one. I have chosen these two 

 neuromeres (IV and V) for description, since they with neuromere VI 

 1 The head somites, likewise, appear at this stage most clearly differentiated. 

 It is, in fact, the " acranial stage " of the embryo. 



