The Nervous System of Amblystoma. 461 



ment although a single egg is rarely found which shows them 

 clearly all at one time as they are shown in the diagram. The 

 two halves of the plate for example often develop unequally and, 

 moreover, the order of appearance and disappearance of the 

 various structures varies greatly in different eggs, and again 

 there are a few transitory structures which in some eggs may 

 never be prominently developed. The confusion caused by 

 these three factors has been carefully considered in the descrip- 

 tion of the preceding stages. It is evident that there are a few 

 important general factors underlying the course of the develop- 

 ment of the neural plate; the markings on the surface of the 

 embryo take the form of grooves and ridges; these grooves and 

 ridges run transversely and longitudinally marking off transverse 

 and longitudinal zones. Such a conception of zones will be made 

 clearer by the following review of the various stages in the 

 development of the plate. 



The embryonic area is first divided longitudinally by a series 

 of grooves, the anterior germinal depression (agd, fig. 5, F), the 

 posterior germinal depression (pgd), and the blastogroove (bg), 

 all three of which develop in the median line of the embryo in 

 the order named. Then to the right and left of the median 

 line develop the low wide ridges of the neural plate which is 

 bounded by the peripheral groove (pg). Outside of the peri- 

 pheral groove another ridge, higher and narrower, develops to 

 form the neural crest (nc). Later the first three grooves dis- 

 appear, except the anterior portion of the anterior germinal 

 depression, and their place is taken by the neural groove which 

 is not shown in the diagram. The transverse zones appear 

 after the longitudinal zones. First the groove which is called 

 the transverse cephalic groove (teg) divides the procephalic 

 lobes from the rest of the plate, then the fourth neuromere (n4) 

 is formed, then the first three neuromeres appear, not following 

 any regular order. In a few eggs of later stages neuromeres 

 appear behind the first four of the procephalic lobes but their 

 history cannot be traced in Amblystoma nor even their number 

 and arrangement determined. In this list the blastopore, the 

 retinal spot (rs), the lateral infolding (li) and the infundibular 



