THE OEIENTATION OF THE FROG's EGG. 385 



of the white are also smaller than those opposite to them. 

 Sections of the hardened eggs, made with a scalpel through 

 the plane of these smaller cells and their opposites, showed that 

 these cells are n6t only smaller superficially, but in the third 

 dimension as well. 



Undoubtedly, then, from the eight-celled stage onwards the 

 distribution of larger and smaller cells on the dark and light 

 sides of the egg is present, and I have been able to push back 

 a step farther the differences noted for the later stages in the 

 preceding section. 



Whether or not a still more careful examination of very 

 favorable material would find the same difference present in 

 the four-celled stage I am unable to say, but it seems not 

 improbable that such a difference exists. 



A study of the method of gastrulation of the egg of the 

 unknown species shows that the first traces of the blastopore 

 appear on the light side of the egg within the white cells. 

 Presumably the pigment has here also extended farther over 

 the sides of the egg than at first. The outlines of the cells in 

 the region of the blastopore are at first polygonal. Dark 

 pigment appears in the walls of the cells, producing the dark 

 line seen in surface view. Certain of the cells pull in from 

 the surface, leaving only their outer small pigmented ends 

 exposed. These cells subsequently pull in all together to 

 form the beginning of the archenteron by invagination. 

 The cells dorsal to the blastopore become narrow and elongated 

 from above downwards. The light cells, below the point of 

 invagination, retain their polygonal outline. 



The changes that take place in the overgrowth of the dorsal 

 lip of the blastopore will be recorded below. First let us 

 examine the embryo when first outlined on the egg. Some- 

 times the outlines of the medullary folds may appear before 

 the yolk-plug has entirely disappeared — at other times not 

 until after this change has taken place. Careful measure- 

 ments of the embryo at this time show that the embryo ante- 

 rior to blastopore covers in length about one third of the 

 periphery of the egg. The relative length of the embryo 



