Page:Popular Science Monthly Volume 85.djvu/124

120 of substances and the differentiation of cells is not so evident in the last named animals because these substances are not so strikingly colored. Indeed the segregation and isolation of different protoplasmic substances in different cleavage cells occurs during the cleavage of the egg in all animals, though such differentiations are much more marked in some cases than in others.

 Styela, showing the cell lineage of various organs, and the distribution of the different kinds of protoplasm to these organs. Muscle cells are shaded by vertical lines, mesenchyme by horizontal lines, nervous system and chorda by stiples.

This same type of cell division, with equal division of the chromosomes and more or less unequal division of the cell body continues long after the cleavage stages—indeed throughout the entire period of embryonic development. Sometimes the division of the cell body is equal, the daughter cells being alike; sometimes it is unequal or differential—