1 68 COMPARATIVE ANATOMY 



this epithelium becomes two-layered, the outer and thinner layer being 

 the periderm. 



By the continued multiplication of the basal cells, the number of 

 layers gradually increases, until by the fourth month, all four layers of 

 the thicker parts of the adult skin have appeared. The cells of the 

 stratum comeum contain a fatty or waxy substance, which helps to form 

 the pasty vemix caseosa which covers the body of the new-born infant. 

 Developing hairs, instead of penetrating this layer, lift it as a continuous 

 sheet, the epitrichial layer. 



Cerium. In most parts of the body, the mesenchyma which produces 

 the corium is derived from cells which have migrated from the parietal 

 layer of the mesoderm. For this reason, that part of the epimere which 

 forms the corium is called the dermatome. Some observers, however, 

 assert that in mammals the entire epimere forms muscle and there is no 

 dermatome in embryos of the group. Moreover, in some vertebrate 

 embryos, if not in all, the ectoderm also contributes to the mesenchyma 

 of the head and possibly, therefore, to the corium. 



Embryonic mesenchyma consists of scattered, stellate cells, separated 

 by wide spaces. It becomes the connective tissue of the corium by secret- 

 ing intercellular fibers, both elastic and non-elastic. By the fourth 

 month, the compact fibrous layer of the corium is distinguishable from 

 the loose areolar tissue under it. Blood vessels and nerves invade the 

 corium from below, hairs and glands grow into it from the epidermis. 

 Abnormalities in the distribution of blood vessels cause birthmarks. 



FINGER-PRINTS 



In all primates, the entire surface of the palms and soles, but no other 

 portion of the body, is marked with fine parallel ridges separated by 

 equally fine grooves. 



At definite places on hands and feet, these ridges form concentric 

 lines or loops. Eleven distinct "friction-ridge patterns" have been 

 distinguished, five on the finger tips, four at the base of the fingers, two 

 near the wrist or ankle. Those of the finger tips are most familiar, 

 since they are used for identification. Since no function has ever been 

 proved for these designs, their presence and their constant position has 

 stimulated much interest and discussion. To useless organs, the hypothe- 

 sis of special creation gives no clue. Are they, then, rudiments of struc- 

 tures functional in the lower animals? See Fig. 126. 



The significant fact about these patterns is that they match precisely, 

 both in number and position, the concentric rows of horny scales on the 

 foot-pads of insectivores, a group which, for various reasons, is thought 

 to be near the direct line of man's ancestry. In the insectivores, the 

 position and arrangement of the scaly ridges is clearly adaptive and 



