The Cellular Basis 



191 



ever it can be recognized in the earlier stages it is probable that the 

 bilateral symmetry of the egg becomes the bilateral symmetry of 

 the developed animal. 



(c) Inverse Symmetry. In most animals bilateral symmetry 

 is not perfect, certain organs being found on one side of the mid 

 line and not on the other, or being larger or differently located on 

 one side as compared with the other ; among all such animals var- 

 iations occasionally occur which show a complete reversal of these 

 asymmetrical organs, i.e., in man the heart and arch of the aorta 



FIG. 68. OUTLINES OF THE UNFERTILIZED EGG OF A SQUID, Loligo, show- 

 ing the polarity and symmetry of the egg with reference to the axes of 

 the developed animal; d, dorsal; v, ventral; /, left; r, right; a, anterior; 

 p, posterior. (After Watase.) 



FIG. 69. MEDIAN SECTION THROUGH EGG OF A FLY, Musca, just after fer- 

 tilization, showing the relations of the polarity and symmetry of the egg 

 to the axes of the developed animal; the long axis of the egg corresponds 

 to the antero-posterior axis of the animal; d, dorsal; v, ventral; m, micro- 

 pyle through which sperm enters the egg; g, glutinous cap over the micro- 

 pyle ; r, polar bodies ; p. egg and sperm nuclei ; do, yolk ; k, peripheral layer 

 of protoplasm; dh, vitelline membrane of egg; ch, chorion. (After Kor- 

 schelt and Heider.) 



