526 LOCV. [Vol. XL 



shows the same embryo with the overlying tissues removed 

 from the brain walls. It is clear from a comparison of these 

 two figures that the line of metameres seen from the surface 

 view arc those occupying the upper part of the neural tube, 

 and that below them the entire neural tube is divided into 

 corresponding segments. The segments do not reach across 

 the median plane, but they are alike in number and position 

 on both sides of the tube. 



Fig. 43 represents an embryo in which the auditory vesicle 

 is just forming. The embryo was intermediate in age between 

 those shown in Figs. 41 and 34. 



Fig. 44 shows a dissection of an embryo of the same age 

 as the one represented in surface view in Fig. 34. 



Fig. 45 is taken from an embryo just younger than that 

 represented in Fig. 35. By the growth of the dorsal wall of 

 the hind-brain the line of metameres have been forced apart. 

 This expanded dorsal wall, being a new growth, is not divided 

 into segments. 



Fig. 46 represents a slightly older embryo with the optic 

 vesicle and also the auditory vesicle removed. There are in 

 this figure eight segments in the hind-brain that show plainly, 

 and faint indications of a ninth. 



Fig. 47 represents a dissection of the embryo from which 

 Fig. 35 was made, and therefore a direct comparison can be 

 made between the two figures. 



Fig. 48 shows the condition of the brain walls in an embryo 

 just older than that represented in Fig. 37, From the continued 

 growth of the dorsal wall the metameres on each side have be- 

 come widely separated. The ear-capsule has not been removed. 



Fig. 49 represents a slightly older embryo than the fore- 

 going one. The auditory and the optic vesicle have been 

 removed. There are now distinctly nine segments in the hind- 

 brain region. The U-shaped segment. No. 12, in the hind- 

 brain lies opposite the ear-capsule. This figure shows well the 

 condition of the neuromeres described in Fig. 37, in which 

 there is no longer (as in earlier stages) a correspondence be- 

 tween the ridges on the upper margin and those on the lower 

 margin of the segmented lateral bands of the neural tube. 



