THE WAY LIFE BEGINS 67 



of the corals, sponges, and star-fish. Indeed the first steps in 

 making the embryo, the multiplication of cells, the mulberry- 

 like ball which results, the hollowing out of the ball, and the 

 cupping in of one side as one would press in a hollow rubber 

 ball, to make the alimentary canal or stomach, and the for- 

 mation of the two resulting layers are the same for all crea- 

 tures from sponges to man. Some idea of these early stages 

 in the formation of animal bodies may be had by a study of 

 the frog's development (Figure 2, page 36). 



The relation of the fertilized ovum to its environment, on 

 the other hand, sometimes floating in the water, now shell 

 covered — or again nestling in the walls of the uterus, differs 

 greatly. The protective envelops or membranes surrounding 

 the growing embryo differ also in birds and mammals. 



It is essential to remember that human life, in common 

 with animal life, starts from a single fertilized egg or stem 

 cell. Biologically, this means, when taken in connection with 

 evidence from many other sources, that the sex cells were 

 once independent, free-swimming beings — all of life that 

 then existed — and later, when they gave rise to many celled 

 bodies, they retained the power of reproducing themselves and 

 their bodies. Through them, it naturally follows, all inherit- 

 ance is transmitted, rather than directly from body to body. 

 It is interesting to know that sex cells, those that go to make 

 the ovaries and the testes of the new individual, are set aside 

 soon after the first cell divisions begin, which indicates how 

 closely these cells are linked together as they pass from genera- 

 tion to generation of the bodies which they build. 



Growth of the Human Embryo Compared 

 with that of the Rabbit 



We have seen that the fertilized ovum, the first cup-shaped 

 (gastrula) structure, and the two primitive layers of cells 

 (a third layer is introduced between the other two in embryos 

 of animals above sponges) are the same for all animals. With 

 the advance to the higher back-boned forms the resemblance 

 continues, as the digestive and excretory (the kidneys and 

 the reproductive organs are included here) tracts are elabo- 



