PRINCIPLES OF HUMAN DEVELOPMENT 293 



ternal blood thus builds up an immunity against the Rh-positive blood that 

 is flowing in the embryo or fetus. The anti-Rh substance diffusing back across 

 the placenta causes destruction of blood cells and blood-forming tissues in 

 the fetus. The first child may escape the harmful effects of anti-Rh substance, 

 but the second fetus is exposed to the substance earlier in development and 

 in higher concentration and may be stillborn or exhibit an acute anemia. If 

 the child is born alive, a transfusion of Rh-negative blood may cause it to 

 survive. Blood, of course, may be typed for the Rh factor, just as it is for other 

 blood factors. 



Principles of human development 



The foregoing discussion of human development was concerned chiefly 

 with a description of the progressive changes in the embryo with time. We 

 shall now consider the question of the principles operating during human 

 development. Are they the same as in other animal forms? Although actual 

 experiments are lacking, observations indicate that the principles are the same. 



The first principle of embryonic development is that of labile organiza- 

 tion in the egg. This principle certainly holds true for the human egg. Identi- 

 cal twins, in which every detail is identical in each of the two individuals, 

 leave no doubt that the early stages of embryonic development of the human 

 show a labile organization of the same type as the sea urchin and the am- 

 phibian egg. Siamese twins fill in the gap, reproducing exactly the result 

 obtained by constricting the amphibian egg. Since the egg shows labile or- 

 ganization, the parts of the egg must be organized by some special region 

 which acts as an organizer. What is the organizer in the human egg? To 

 answer this question let us review the early stages of human development and 

 compare these with similar stages in the chick embryo. 



The human embryo develops from an inner mass of cells by first forming 

 a primitive streak (Fig. 174). The notochord of the embryo develops from 

 the anterior end of the primitive streak, and the neural plate forms above 

 the notochord. Now since we know by actual experiment that the anterior 

 end of the primitive streak in the chick embryo induces a nervous system in 

 the overlying ectoderm, we are justified in drawing the tentative conclusion 

 that the anterior end of the primitive streak in the human embryo also in- 

 duces the primitive nervous system. The evidence is based on the homology 

 of the primitive streak of mammals with that of birds. A second principle 



