Page:Popular Science Monthly Volume 84.djvu/534

530 activities, reactions, instincts, habits, and intelligence of the mature animal.

Is not this miracle of development more wonderful than any possible miracle of creation? And yet as one watches this marvellous process by which the fertilized egg grows into the embryo, and this into the adult, each step appears relatively simple, each perceptible change is minute; but the changes are innumerable and unceasing and in the end they accomplish this miracle of transforming the fertilized egg cell into the fish, or frog, or man—a thing which would be incredible were it not for the fact that it has been seen by hundreds of observers and can be verified at any time by those who will take the trouble to study the process for themselves.

 ''Amphioxus. A, one cell; B, two cells; C and D, four cells; E, eight cells; F, sixteen cells; Q, blastula stage of about ninety-six cells; H, section through the same showing the cleavage cavity; I, blastula seen from the left side showing three zones of cells, viz., an upper clear zone of ectoderm, a middle (faintly shaded) zone of mesoderm and a lower (deeply shaded) zone of entoderm cells; J, section through the same showing these three types of cells; K and L,'' successive stages in the gastrulation; cells indicated as in the preceding figure. In all figures except D the polar body is shown at the upper pole. Figs. A-H after Hatschek; Figs. I-L after Korschelt and Heider and Cerfontain. a, anterior; p, posterior; v, ventral; d, dorsal; bc, blastoccel; gc, gastroccel.