22 EARLY AMPHIBIAN DEVELOPMENT 



In the same way, the great circle at right angles to the egg-axis is 

 the equator of the egg. It coincides more or less with the line of 

 demarcation between the pigmented cells of the animal hemisphere 

 and the Hghter-coloured cells of the vegetative hemisphere; fre- 

 quently, however, the pigment extends well below the equator. 

 Latitudinal position is not so easy to define as longitudinal, since 

 the egg-equator is not clearly marked. In the meridian of sym- 

 metry, however, latitudinal position can be accurately defined as so 

 many degrees above or below the dorsal lip of the blastopore. 



With this in mind, it is now possible to pass to a description of 

 the facts as found in the egg of the Urodele. Most of the cells of 

 the vegetative hemisphere of the blastula eventually get tucked in 

 or enclosed, and find themselves inside the embryo when gastru- 

 lation has been completed. A crescent-shaped region immediately 

 above the position of the dorsal lip of the blastopore, and extending 

 up some way above the equator, is presumptive notochord. On 

 each side of this is a strip which will give rise to mesodermal so- 

 mites and to the unsegmented mesoderm of the lateral plate. Below 

 the latitudinal level of the dorsal lip is a region which includes the 

 yolk-cells of the vegetative pole, and which will give rise to the front, 

 ventral, and lateral walls of the gut-cavity and, eventually, to its 

 definitive roof as well. Most of the ventral half (not to be confused 

 with vegetative half) of the blastula, composed of portions of the 

 vegetative as well as of the animal hemisphere, is presumptive 

 epidermis. This leaves only one region unaccounted for; this, 

 occupying most of the dorsal half of the animal hemisphere (minus 

 the presumptive notochord and mesoderm regions mentioned 

 above), is presumptive neural folds. This latter region may be 

 described in the blastula as a crescent of which the horns extend 

 down the sides of the embryo from the animal pole to the equator 

 along meridians rather more than 90° right and left from the dorsal 

 mid-line. The central part of the crescent extends from the animal 

 pole to the point on the dorsal meridian to which the presumptive 

 notochord region reaches, i.e. about 30° latitude above the equator. 

 It is important to notice that at this early stage, in the blastula, 

 the presumptive neural fold region occupies an elongated region 

 which hes at right angles to the plane of bilateral symmetry.^ While 

 1 Goerttler, 1925; Vogt, 1926 a. 



