Page:Popular Science Monthly Volume 85.djvu/243

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dextral snails (Figs. 42, 43). There is good reason to believe that in such cases the unsegmented egg is also inversely symmetrical as compared with the more usual type (Kg. 42). In all of these cases there is a direct correspondence between the polarity and symmetry of the oosperm and the polarity and symmetry of the developed animal (Fig. 40-44).

 Figs. 42, 43, 44. The cause of inverse symmetry in snails. In each case the right-hand column represents dextral forms, the left-hand column sinistral ones.

In many animals the ectoderm, endoderm and mesoderm may be traced back to areas of peculiar protoplasm in the oosperm, but in addition to this one can recognize in the ascidian egg areas of peculiar protoplasm which will give rise to mesenchyme, muscles, nervous system and notochord, and these substances are present in the oosperm in the