Page:Popular Science Monthly Volume 85.djvu/242

238 eggs are bilateral even before fertilization; in still other cases bilaterality 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

symmetry of the egg bcomesbecomes [sic] the bilateral symmetry of the developed animal.

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 symmetry, 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