REPRODUCTION AND DEVELOPMENT 285 



ently it may be distinguished as a mammalian embryo, but it must 

 progress to still more advanced conditions before one can distinguish 

 the characters that are distinctly human. Here, again, the evidence 

 suggests some sort or path of relationship between Man and other 

 animals. 



Abnormalities. Human interest is usually stirred by the ap- 

 pearance of monstrosities and abnormalities in young animals, par- 

 ticularly in infants. Explanations based on superstitions have some- 

 times been cruel. To the student of Embryology, it is considerably 

 more of a marvel that so many individuals of the same species re- 

 semble each other so closely and that abnormalities are so infre- 

 quent. For, as has been emphasized, protoplasm is highly suscep- 

 tible to all sorts of conditions in its environment, and in addition, 

 the protoplasm of a developing embryo is involved in an accom- 

 plishment that depends on a number of accurately adjusted mechani- 

 cal and chemical events. Cell divisions, growth, diflferentiations, 

 foldings, and changes of position are phases of development that 

 are coordinated with great exactness. It is not difficult to imagine 

 that minor changes in the environment of a developing embryo at 

 a critical stage may effect departures from the usual course of 

 developmental events and produce an abnormal form. When this is 

 put to test by subjecting embryos to abnormal environments, just 

 these results ensue. What we call a normal form in an animal is the 

 result of the usual shifting of layers, of the usual growth of parts 

 in relation to each other, of the spacing of structures, and the usual 

 undercurrent of chemical changes that produce the appropriate 

 tissue at the proper time. 



The most common abnormalities are duplicities, double heads, 

 double posterior parts, or double limbs. The origins of these are 

 to be sought in the properties of the embryonic parts which nor- 

 mally give rise to a single structure. The region or bud that or- 

 dinarily develops into a single limb, is composed of embryonic 

 tissues that possess the general property of limb formation. A region 



