40 



POLARITY AND SYMMETRY; 



e/7 



fe/7/r. 



7b/-J' 



L 



Fig. 14. Egg of a frog, 

 Rana fusca, after the for- 

 mation of the grey crescent 

 igr.h.m.). an: animal side; 

 veg: vegetative side; dors'. 

 dorsal side; ventr: ventral 

 side. After Schleip. 



distributed; often their density increases from the animal 

 toward the vegetative pole. There may also be differences in 

 pigmentation. In many amphibians, for example, the animal 

 region of the egg surface is darkly 

 pigmented, whereas the vegetative 

 region is unpigmented (Fig. 14). 



In the great majority of animals, 

 the unripe eggs {oocytes) possess 

 a polar structure already on leav- 

 ing the ovary. Evidently, the 

 polarity originates during the 

 growth of the egg in the ovary. 

 In many cases a connection can 

 be observed between the polarity 

 of the oocyte, and the way in 

 which it is attached to the ovary. 

 The place of attachment, which is 

 also the place where the food 

 stream from the maternal tissues 



enters the oocyte, is in some groups the future animal pole, 

 in others the future vegetative pole of the egg. 



Apart from polarity, many eggs possess an obvious bilateral 

 symmetry. This, too, may be expressed in the shape itself of 

 the unfertilised egg, as in insects and cuttlefish. In other cases, 

 the symmetry is not visible until after fertilisation. In the eggs 

 of many amphibians, a zone of lighter pigmentation, the so- 

 called grey crescent, develops on one side of the egg along the 

 boundary of the dark animal half in the first few hours after 

 fertilisation. This marks the dorsal side of the egg, and later 

 of the embryo. The opposite side becomes the ventral side 

 (Fig. 14). Once the grey crescent has been formed, we can 

 divide the egg into symmetrical halves in only one way, viz. 

 through the meridional plane which bisects the grey crescent. 

 In normal development this plane, the median plane, will be- 

 come the plane of symmetry of the embryo. It contains the 

 main axis, and also the dorso-ventral axis which is at right 

 angles to the main axis, and connects two opposite points on 

 the equator of the egg. 



