THE NATURAL HISTORY OF MAN 55 



separate masses, by a process of fusion of the segments and 

 then a later splitting, either lengthwise or tangentially, the 

 long, flat muscles in the front and back of the body are formed. 

 Also many muscles shift from their early positions to some 

 quite distant place. This is the case, for example, with the 

 muscular diaphragm, which forms in the neck and is supplied 

 by a nerve from this region but which, during development, 

 moves posteriorly dragging its nerve with it, until in the final 

 position it separates the thorax from the abdomen. All of 

 this differentiation and shifting of muscles has been com- 

 pleted some time before the child is born and therefore is not 

 produced by any functional necessities after birth, but is due 

 to heredity. 



The Limbs. Until the beginning of the fourth week of de- 

 velopment, man is a limbless vertebrate (Fig. 23). The rudi- 

 ments of the limbs then grow out as buds from the side of 

 the body and, as they elongate, five projections appear at the 

 end of each of the buds which represent the developing fingers 

 and toes. This peripheral cleavage into five parts perhaps 

 is representative of the five main segments of the body from 

 which the limb projects. Inasmuch as the five-fingered, or 

 pentadactyl, limb is the common vertebrate type it is believed 

 that polydactylism, or an extra number of fingers or toes, is 

 not a reversion to an ancestral form but is due to an unknown 

 cause. 



The Skeleton. The skeleton is preceded in very early pre- 

 natal life by two kinds of material, cartilage and membrane. 

 While most of the bones of the skeleton are ossified from car- 

 tilage, the bones of the cranium are largely developed from 

 membrane. The cranial bones begin ossification at their 

 centers near the end of the second month of prenatal life, the 

 growth of the cranium being due chiefly to additions at the 

 edges of the bones. In the long bones, like the femur and 

 humerus, which are mapped out in cartilage, the centers of 



