8 ART. 3. S. IKEDA : CONTRIBUTIONS 



the time the lips have entirely encircled the subequatorial region, 

 the former is reduced to ahont two-thirds of its former dimensions 

 (Fig'. 10). 



Between the translucent area above, and the blastopore lips 

 below, a broad opaque band may be recognized encircling the 

 equatorial zone of the egg. It gradually spreads upwards and 

 downwards concurrently with the gradual diminution of the 

 bhistoporic area and of the area of the segmentation-cavity (Figs. 

 9-1 B). This I shall call the " Equatorial Zone." I am inclined 

 to think that it corresponds to the embryonic zone found in the 

 eggs of Selachii, Teleostei and other groups of fishes. It is not, 

 however, so prominent, in normal eggs, as it is said to be by 

 some investigators in other amphibian eggs under the name of the 

 "Embryonic Ring," the "Medullar Ring," or the "Unwachsungs- 

 rände." It is true that in some abnormal eggs, in which the 

 blastoporic area and the area of the segmentation cavity have been 

 disturbed in any way, it may become as prominent as it has been 

 figured by these writers. In such cases, the neural plate may 

 also often become more conspicuous than usual, and the correla- 

 tion which is ordinarily found between the reduction in size of the 

 a ]-en of the segmentation cavity and of the blastoporic area may 

 be lost. By an examination of sectioned eggs, I have assured 

 myself that the equatorial zone is, in the first stage of gastrula- 

 tion, nothing but a simple accumulation of blastoderm-cells around 

 the egg-equator. 



When the equatorial zone has come to occupy about the 

 middle two- thirds of the egg-surface, the neural plate may be faintly 

 detected by two shady lines within the zone, with a slightly 

 projected anterior margin (the projecting tongue of Eycleshymer 

 ['98] found in Amblystema) (Figs. 10 and 11). In some eggs, 



