Page:Popular Science Monthly Volume 85.djvu/244

240 approximate positions and proportions which they will have in the embryo and larva (Figs. 28-31).

Indeed, there are types of localization of these cytoplasmic materials in the egg which are characteristic of certain phyla; thus there are the ctenophore, the flat-worm, the echinoderm, the annelid, mollusk and the chordate types of cytoplasmic localization (Fig. 45). The polarity, symmetry and pattern of a jelly fish, star-fish, worm, mollusk, insect or vertebrate are forshadowedforeshadowed [sic] by the characteristic polarity, symmetry and

 The cells 1a-1d, 2a-2d and 3a-3d give rise to all the ectoderm; 4d or M gives rise to mesoderm; A, B, C, D to endoderm.

pattern of the cytoplasm of the egg either before or immediately after fertilization. In all of these phyla eggs may develop without fertilization, either by natural or by artificial parthenogenesis, and in such cases the characteristic polarity, symmetry and pattern of the adult are found in the cytoplasm of the egg just as if the latter had been fertilized. The conclusion seems to be justified that these earliest and most fundamental differentiations which distinguished the eggs of various phyla are not dependent upon the sperm.