56 THE EVOLUTION OF MAN 



ossification appear about the eighth week of prenatal life in 

 the center of the shaft, and, at birth, while the shaft is bone, 

 both extremities are still cartilaginous. 



The Skin. The outside portion of the skin, or epidermis, is 

 derived from the ectoderm, while the deeper portion, or 

 dermis, is developed from the mesoderm. The human skin 

 at the end of the second prenatal month is translucent and has 

 many points in common with that of fishes and amphibians. 

 In the third month a delicate, superficial, horny stratum ap- 

 pears, a stage which has been held to represent the evolution 

 from an aquatic to a terrestrial form of life. 



Hairs are outgrowths from the epidermis and are developed 

 in groups and lines. Their arrangement can best be explained 

 on the supposition that originally the skin was covered by 

 scales and that the hair grew out in groups at their tessellated 

 junctions as is found in certain of the edentates. The skin of 

 man, in comparison with that of the other primates, is com- 

 paratively hairless, which is probably a recently acquired 

 character. At the seventh month of prenatal life the chim- 

 panzee and gorilla have well-developed hair on the scalp, eye- 

 brows, and lips, while the rest of the body is covered with fine 

 hair. This is also the condition of the human fetus at a corre- 

 sponding period. The hair slopes in man are also very similar 

 to those of the apes. Some hairs, such as those present in the 

 eyebrows, perhaps originally had a sensory function and, in 

 general, hair appears to be a modification of certain glandu- 

 lar and sensory structures found in the skin of the amphibians. 

 Evidence has accumulated which shows that the development 

 of the hair is regulated, at least to some degree, by an internal 

 secretion of the thyroid and sex glands. 



Face and Nose. The face is developed from a series of 

 paired processes surrounding the primitive mouth (Fig. 21). 

 These grow in from the sides toward the mid-ventral line 

 where they normally fuse by the third prenatal month, by a 



