53o 



THE POPULAR SCIENCE MONTHLY 



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 proc- 

 ess for themselves. 



Fig. 10. Successive Stages in the Cleavage and Gasteulation of Amphio&us. 

 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 ; /, 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 indi- 

 cated 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 Cer- 

 fontain. a, anterior ; p, posterior ; v, ventral ; &, dorsal ; he, blastoccel ; gc, gastroccel. 



