THE NATURAL HISTORY OF MAN 57 



method somewhat akin to the healing of wounds, to form the 

 nose and lips. A failure of proper fusion of these processes 

 results in harelip and various other facial deformities. 



In the development of the human nasal cavity a recapitula- 

 tion of the conditions present in the various classes of verte- 

 brates is to be noted. At the end of the third week of fetal 

 life the rudiments of the olfactory organ appear as two thick- 

 ened plates of ectoderm which are in contact with the under 

 surface of the fore-brain. A week later these plates become 

 depressed and are found at the bottom of the olfactory pits. 

 This condition is similar to that found in fishes. Two weeks 

 later the depression has deepened and a nasal cavity is formed 

 which is somewhat similar in structure to that found in the 

 air-breathing or lung fishes. By the seventh week this primi- 

 tive nasal cavity has rapidly enlarged and a communication 

 has been made with the mouth as in amphibians. The mam- 

 malian condition is reached by the third month. At this time 

 the roof of the mouth is completed by the growth to the center 

 and fusion of two lateral shelves. A failure of these shelves 

 to unite produces cleft palate. 



The teeth are formed by local outgrowths of the ectoderm 

 and mesoderm lining the mouth, the enamel being formed 

 from the ectoderm, and the dentine and pulp from the meso- 

 derm. The human molars with their several cusps have 

 evolved from the simple conical teeth of fishes and reptiles 

 either by a process of partial fusion or by an outgrowth of 

 cusps from the conical tooth. 



The Prenatal Nourishment of the Embryo. During pre- 

 natal life the fetus is enclosed in a fluid-filled sac, the wall of 

 which is composed of two membranes, the inner called the 

 amnion and the outer the chorion, which serve for protection 

 and nutrition (Fig. 12). The development of an embryo in 

 nt-ero is dependent upon its ability to secure nourishment and 

 to eliminate certain wastes. In order to do this it must get 



