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, i.e., the germinal disc or yolk, that indicates it. 



This predelineation of embryonic axes within the unsegmented 

 ovum has been interpreted physiologically as due to gradients in 

 rate of metabolic processes along the embryonic axes (Child), 

 which determine the localization of the main developmental 

 events. 



