CHAPTER IV 



Polarity and symmetry; 

 Gradient-fields 



In the previous chapter we have seen, that, in broad outlines, 

 the newly fertilised egg is to be regarded as a homogeneous 

 system, and that its multiplicity is only intensive, not extensive. 

 However, we shall now discuss a number of phenomena that 

 show that this is true only with certain restrictions. 



Even a superficial examination shows that the egg can not 

 be so entirely homogeneous as is, for instance, a water drop, 

 because we can distinguish between egg cytoplasm and nucleus. 

 But, apart from this, a further organisation is demonstrable in 

 the egg; this is expressed in its polarity and its symmetry. 



All animal eggs have a polar structure, i.e. two opposite 

 poles can be distinguished, called the animal and vegetative 

 poles respectively. They are connected by the main axis of the 

 egg. Considering the egg as a globe, we may call all planes 

 that contain the main axis meridian planes, whereas the plane 

 that bisects the main axis at right angles is the equatorial plane. 

 In some cases, e.g. in the oblong eggs of insects (Fig. 26) and 

 cuttlefish, the polarity is revealed in the shape itself of the 

 egg. In other cases, the egg is more or less spherical, but its 

 polarity is evident in other ways: the polar bodies are given 

 off at the animal pole of the egg (p. 10) ; after fertilisation, 

 the zygote nucleus often lies in an eccentric position, nearer 

 to the animal pole. The first two cleavages nearly always take 

 place in meridional planes so that they intersect in the main 

 axis of the egg. 



Further the polarity is often apparent in the arrangement 

 of the inclusions of the egg cytoplasm. In many eggs, the food 

 substances which together constitute the yolk are not evenly 



