THE NATURAL HISTORY OF MAN 43 



cavities develop (Fig. 12), one becomes the amniotic cavity 

 and ultimately surrounds the embryo, and the other forms the 

 yolk sac, part of which eventually becomes the alimentary 

 canal. Some of the cells of the two sacs, which lie adjacent 

 and are destined to develop the embryo, form two layers, 

 known as the ectoderm and endoderm, or the external and 

 internal membranes. This structural arrangement represents 

 the earliest stage known in man's development, the previous 

 stages described above being hypothetical and based on obser- 

 vations made on the developing eggs of other mammals. 

 Later, through the multiplication of the ectoderm cells an 

 elongated area, known as the primitive streak, is formed and 

 this indicates the anteroposterior axis of the embryo. 



Along this primitive streak, between the outer ectoderm and 

 the inner endoderm layers, a third layer is produced, known 

 as the mesoderm. We now have in the region where the em- 

 bryo is developing, known as the embryonic area, three 

 layers of cells, each having its own distinctive characteristics. 

 These are known as the primary germ layers, and from them 

 all the organs and parts of the body are later derived. In the 

 human embryo all of the organs are formed by the third month 

 of development. From the outer layer, or ectoderm, are 

 formed the outer layer of the skin, or epidermis, including its 

 various appendages such as the hair and sweat glands, the cells 

 lining the mouth, the enamel of the teeth, and the entire 

 nervous system including the sensory portions of the sense 

 organs. From the middle layer, or mesoderm, are formed the 

 skeleton and other supporting tissues and the muscles, the 

 vascular system, and the sex cells. From the innermost layer, 

 or endoderm, are developed the cells lining the alimentary 

 canal and the essential secreting cells of the various organs 

 which develop as outgrowths from it, such as the thyroid 

 gland in the neck, the lungs, the liver, and the pancreas. In 

 general it may be said that the endoderm supplies the ali- 



