468 Leland Griggs. 



although it shows the weakness of the old theories does not 

 present any exact point that may safely be taken as the anterior 

 limit of the adult brain. 



Before going further into a discussion of the neuromeres of this 

 stage it will be necessary to describe the origin of the landmarks of 

 the adult brain. As soon as the neural crests begin to close a con- 

 striction appears in the neural canal marking off the primary 

 fore-brain vesicle (fb, fig. 8). Then a second constriction appears 

 marking off the mid-brain vesicle (ol, fig. 8). When once these 

 landmarks have been identified they may be readily traced back 

 to embryos as young as those shown in fig. 8, but they cannot be 

 seen in any younger specimens. This process of the development 

 of the two primary brain vesicles becomes very clear by tracing 

 the series backward in the following order — fig. 9 B, fig. 9 A, 

 fig. 8 B, fig. 8 A. A third landmark ,the first division or segment 

 of the hind-brain vesicle, appears in the neural crest (c, fig. 8.) 

 In its later stages (c, fig. 9) it appears as the first hind-brain 

 neuromere or cerebellar neuromere of the closed neural tube. 

 The cerebellum arises from its roof. 



The relation of the neuromeres to these three well recognized 

 landmarks is clearly a matter of prime importance but its impor- 

 tance may be overestimated as compared with the relation of 

 the whole group or tagma of procephalic neuromeres to the 

 parts of the adult brain. This tagma has been shown to be older 

 then the individual neuromeres and to be more prominent and 

 regular in its appearance. The first problem is to determine 

 the relation of the tagma as a whole to the primary brain vesicles; 

 the relation of the individual neuromeres to these vesicles or to 

 other later landmarks may prove to be a different and more 

 difficult problem. It is possible that in Amblystoma in which 

 the segmentation of the neural plate is somewhat rudmientary 

 the original number of neuromeres in each tagma may be subse- 

 quently changed by fusion, or by temporary suppression. Be 

 that as it may, it is better to consider first the relation of the 

 procephalic lobes as a whole to the adult brain. 



The posterior boundary to these lobes has been shown to be 

 the great transverse cephalic groove, the first landmark to appear 



