1 GROWTH OF THE GERM-CELLS 7 



side to the other, a gradual increase of yolk in passing in the 

 opposite direction. Since the egg is spherical one line, which 

 is different from all others, may be drawn through the centre 

 of the protoplasmic portion at the surface, the centre of the 

 egg itself, and the centre of the yolk portion *at the surface. 

 This line is the egg-axis, and its ends or poles are obviously 

 unlike, the protoplasmic pole being known as the animal, the 

 yolk-pole as the vegetative. The equator of the egg is the 

 plane passing through the centre at right angles to the axis. 

 It will be evident that since the cytoplasm and the yolk are 

 symmetrically distributed round the axis, the egg may be 

 divided into precisely similar halves by any plane that 

 includes the axis, but by none other. In other words, the egg 

 is radially symmetrical about its axis. 



This radial symmetry is further emphasized by the arrange- 

 ment of the pigment, which is deposited in the form of minute 

 granules in a superficial layer in the animal portion and 

 extends some little way, to a variable extent in different eggs, 

 below the equator into the vegetative hemisphere. The 

 boundary between pigmented and unpigmented portions is 

 thus a circle parallel to the equator. Lastly, the nucleus 

 the germinal vesicle in the immature egg, the female pro- 

 nucleus in the mature egg, and the two pro-nuclei in the 

 fertilized egg lies in the axis, but excentrically, in the animal 

 hemisphere. 



As a result of fertilization, however, this primitive radial 

 symmetry is lost, and replaced by a bilateral symmetry, for, 

 opposite the point of entrance of the spermatozoon (which is 

 somewhere in the animal hemisphere), there appears on the 

 border of the pigmented area a grey crescent (Fig. 3), due to 

 the disappearance of the superficial pigment into the interior. 

 The egg is now divisible into similar halves only by a plane 

 including the axis and passing through the middle point of 

 the grey crescent, and about this plane it is now bilaterally 

 symmetrical. 



We shall see that this plane of symmetry of the fertilized 

 but unsegmented egg is approximately coincident with the 

 median plane of the embryo, since the side of the grey 



