456 Leland Griggs. 



typical eggs showing the first and last steps in the process, while 

 some of the intermediate steps are shown in fig. 5. Plate I, 

 figs 8 and 9 show the faint appearance of one neuromere. These 

 two embryos are especially valuable because they show, although 

 faintly, the anterior and posterior germinal depressions and 

 hence it is possible to locate the neuromere exactly. It lies 

 opposite the posterior end of the anterior depression as can be 

 seen plainly in Plate I, fig. 9. Plate I, fig. 10 shows all four 

 neuronieres. The first and most anterior one is the faintest 

 and is rather difficult to see in a photograph. Plate I, fig. 11 

 shows the neuromeres as seen from the side. Their shaded 

 posterior margins appear as dark bands, the first neuromere 

 being very faintly marked off from the second. While it is very 

 difficult to show the details in photographs they are offered as 

 a general confirmation of the more exact history of the neuro- 

 meres as shown by drawings. This follows the convincing method 

 used by Locy ('95) in his treatment of the subject of neuromeres 

 in the embryo of Acanthias. 



This brief history of the development of the important pro- 

 cephalic neuromeres should be supplemented by a description 

 of some details of minor importance. The neuromeres are 

 usually not all of equal size. The fourth neuromere, the first 

 to appear in point of time, is often very prominent (n4, fig. 4, B, 

 fig. 5, D) and the second, although never higher than the others 

 is sometimes much wider (n2, fig. 4, B, fig. 5, D). The first 

 neuromere is usually the most faintly marked of all. Variations 

 from the typical pattern are common particularly in the direc- 

 tion of so called ''hemi-embryos" (fig. 5, B, E). Neal ('98) has 

 used this fact of lack of correspondence between right and left 

 sides as an argument against Locy's theory of neuromeres. 

 Such a condition, at least in Amblystoma, seems to show rather 

 that the right and left sides develop more or less independently. 

 After the full development of the neurome:es there is an entire 

 correspondence between the two sides as will be shown in the 

 next stage. Very rarely there are indications of more than the 

 usual number of neuromeres. Among the hundreds of eggs 

 examined there were only two such specimens found one of 



