INTRODUCTION 15 
name; they often furnish the only clear indication of polarity 
before cleavage begins. 
With reference to the heteropolar ovic axis a series of meridia 
may be defined, drawn from pole to pole over the surface; likewise 
an equator and a series of horizontal zones parallel to the equator. 
Thus directions on the surface of the ovum may be defined as 
meridional, equatorial, or oblique. 
Cleavage takes place with reference to the axis of the ovum. 
Thus in holoblastic vertebrate ova the first and second cleavage 
planes are meridional, and the third usually equatorial. The 
mammalian ovum may form an exception to this rule, though 
little is known, as a matter of fact, about the polarity of the mam- 
malian ovum. The cleavage of meroblastic ova takes place 
likewise with reference to the polarity (see Chap. II); and the 
location of the primary germ-layers is determined by the polarity. 
Not only is the ovum heteropolar, but in many bilateral 
animals, and perhaps in all, it is bilaterally symmetrical before 
cleavage begins; that is to say, one of the meridional planes 
defines the longitudinal axis of the future embryo, and the direc- 
tion of anterior and posterior ends is also predetermined in this 
meridian, so that halves of the egg corresponding to future right 
and left sides of the embryo may be distinguished. In the frog’s 
egg the plane of symmetry is marked by a gray crescent that 
appears above the equator on the side of the egg that corresponds 
to the hinder end of the embryo. This crescent is bisected by 
the meridional plane of symmetry. In the hen’s egg the plane 
of symmetry of the embryo appears on the surface of the yolk 
in a line at right angles to the axis of the shell, and the left side 
of the embryo is turned towards the broad end, the right side 
towards the narrow end of the shell. The same plane of sym- 
metry must exist in the ovum prior to cleavage for reasons ex- 
plained beyond, although there is no morphological differentiation 
in the ovum proper, 7.e., the germinal disc or yolk, that indicates it. 
This predelineation of embryonic areas within the unseg- 
mented ovum has led to the idea that the ovum contains various 
materials, so-called formative stuffs, in typical arrangement, that 
determine in some physiological way the formation of specific 
structures. 
