172 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



neuromeres never shift their position, the only change being a gradual 

 assumption, by each of the local thickenings, of an inner concavity in 

 the region of the lateral zones. 



Frontal sections of an embryo 15 mm. long show that dorsally all 

 traces of the neuromeres are lost. A frontal section in the region of 

 the lateral zones from an embryo of this stage is represented in Figure 

 30, Plate 5. A great separation of the lateral walls of the medulla is 

 seen to have taken place in the region of neuromeres III, IV, and V. 

 At this stage only do the neuromeres possess the characteristics de- 

 scribed by Orr for the Lizard (see page 1G7). While the external con- 

 strictions are only faintly shown, owing to the increase of the " white 

 substance " on the sides of the medulla, the internal ridges and con- 

 cavities are well marked. From this stage onward the neuromeres 

 begin to disappear. In embryos of 40 mm. to 50 mm., neuromere VI, 

 in relation with the facialis nerve, is the most clearly marked of tlie 

 neui'omeres. 



Before passing to an examination of the evidence of neuromeres in the 

 trunk region, I wish to emphasize the fact that the hindbraiu neuro- 

 meres cannot be regarded as structures dependent upon the pressure of 

 mesodermal somites. Being local thickenings of the lateral wall of the 

 neural tube they are obviously inexplicable on such a simple mechanical 

 basis. They are structural differentiations of the tube in regions where 

 the mesoderm has not yet extended, — that is, in the dorsal and lateral 

 portions of the tube, the mesoderm of the head being still ventral in re- 

 lation to the neural tube. 



c. Summary. 



In the preceding study of the hindbraiu neuromeres in S. acanthias, 

 I have supplemented Orr's criteria (applicable to later stages) by a de- 

 scription of the structure of the neuromeres in Squalus in earlier stages 

 of development, i. e. in embryos of 14-50 somites. The characteristics 

 possessed by hindbrain neuromeres in these earlier stages may be sum- 

 marized as follows. Each neui-omere is separated from its neighbor by 

 an external constriction, which passes entirely around the neural tube. 

 There is dorsally and ventrally an mternal ridge corresponding to this 

 external constriction ; but the ridge vanishes in the region of the lateral 

 zones, being replaced by an internal depression or groove. The nuclei 

 of the lateral wall are, however, still arranged (Fig. 37) in a manner 

 which approximates that of the region of the internal ridges, notwith- 

 standing that the thickening of the lateral wall of the neuromere has 



