THE PRIMARY SEGMENTATION. 



that the extreme anterior end of the vertebrate brain still bears 

 the marks of primitive segmentation. This, as Dr. Whitman 

 has shown, is also the case with the invertebrates. 



In Fig. 6, the auditory vesicle is formed. The neural seg- 

 ments of the fore- and mid-brains are no longer discernible 



FIG. 6. Head of embryo after the formation of the auditory vesicle, the first five head segments no 

 longer distinguishable, au, auditory vesicle, na, nasal epithelium, ntb, mid-brain. 



from surface view, but those of the hind-brain, beginning with 

 No. 6, are clearly seen. The neuromeres of that region are 

 now in contact in the median plane ; soon, by the lateral growth 

 of the upper brain wall, they become separated as shown in Fig. 

 7. This is the stage in which neural segments have been here- 

 tofore described. They have been designated neuromeres. The 



FIG. 7. Head of embryo somewhat older than the one in Fig. 6. nv, beginning of the fifth nerve. 

 Other reference marks as before. 



