Page:Popular Science Monthly Volume 85.djvu/246

242 certain influences have come from the nucleus of the egg, and perhaps from the egg chromosomes. There is no doubt that most of the differentiations of the egg cytoplasm have arisen during the ovarium history of the egg, and as a result of the interaction of nucleus and cytoplasm;

 cross-hatched area, mesoderm or mesenchyme (mes); horizontal lines, endoderm (end); clear area, ectoderm (ect). In the first four figures the pattern of localization Is that which is found at the close of the first cleavage; in the annelid egg the localization of later stages is projected upon the egg.

but the fact remains that ''at the time of fertilization the hereditary potencies of the two germ, cells are not equal, all the early stages of development, including the polarity, symmetry, type of cleavage, and the pattern, or relative positions and proportions of future organs, being foreshadowed in the cytoplasm of the egg cell, while only the differentiations of later development are influenced by the sperm. In short, the egg cytoplasm fixes the general type of development and the sperm and egg nuclei supply only the details.''

We are vertebrates because our mothers were vertebrates and