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THE POPULAR SCIENCE MONTHLY 



eggs are bilateral even before fertilization; in still other cases bilater- 

 ality does not become visible until later in development and we do not 

 now know whether it is present in earlier stages or not; but wherever 

 it can be recognized in the earlier stages it is certain that the bilateral 



Fig. 40. Fig. 41. 



Fig. 40. Outlines op the Unfertilized Egg of a Squid, Lollgo, showing the 

 polarity and symmetry of the egg with reference to the axes of the developed animal ; 

 d, dorsal; v, ventral; I, left; r, right; a, anterior; p, posterior. (After Watase.) 



Fig. 41. Median Section through Egg of a Fly, Musca, just after fertilization, 

 showing the relations of the polarity and symmetry of the egg to the axes of the de- 

 veloped animal ; the long axis of the egg corresponds to the antero-posterior axis of 

 the animal ; d, dorsal ; v, ventral ; m, micropyle through which sperm enters the egg ; 

 g, glutinous cap over the micropyle ; r, polar bodies ; p, egg and sperm nuclei ; do, 

 yolk ; k, peripheral layer of protoplasm ; dli, vitteline membrane of egg ; ch, chorion. 

 (After Korschelt and Heider.) 



symmetry of the egg bcomes 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 variations occasionally occur which show a 

 complete reversal of these asymmetrical organs, i. e., in man the heart 

 and arch of the aorta may occur on the right side instead of the left, 

 the pyloris and chief portion of the liver on the left instead of the 

 right, etc. Among certain snails this inversion of symmetry may occur 

 regularly in certain species and not in others, the inverse form being 

 known as sinistral and the ordinary form as dextral (Fig. 44). In 

 these sinistral snails, and probably in all animals showing inverse sym- 

 metry, the embryo is inversely symmetrical and every cleavage of the 

 egg from the first to the last is the inverse of that which occurs in 



