4. By the conversion of the one-layered blastula into two layers of cells, the Gastrula stage of the embryo is attained. The Gastrula consists of two layers of cells surrounding a central cavity, which is the Archenteron, or the body-cavity that will hold the intestines. During the past twelve years many specimens of human gastrulas have been observed. The earliest form was that seen in 1908 by Teacher and Boyce.[35] This embryo was 1.95 mm. in length by 0.95 mm. in width, about twice the size of a pin-head. It showed on section the endoderm, the ectoderm, and the beginning mesoderm, enclosed in a spherical mass of trophoblastic cells. The mesoderm is a plate of cells lying between the endodermic and ectodermic plates. When the mesoderm develops into two plates, a cavity, called the Primitive Coelom, appears between the plates. The Coelom becomes the space between the viscera and the body-walls in later development. [35] _Contributions to the Study of the Early Development and Embedding of the Human Embryo._ Glasgow, 1911. From the primary embryonic layers of cells, the ectoderm, the endoderm, and mesoderm, all the parts of the body are built up. From the ectoderm are produced the skin, nails, hair, the epithelium of the sebaceous, sweat, and mammary glands, the epithelium of the mouth and salivary glands, the teeth-enamel, the epithelium of the nasal tract, of the ear, of the front of the eye, and the whole spinal cord and the brain, with their outgrowths. From the endoderm come the epithelium of the respiratory tract, of most of the digestive tract with the liver and pancreas, the epithelium of the thyroid body, the bladder, and other minor parts. From the mesoderm are developed bone, dentine, cartilage, lymph, blood, fibrous and alveolar tissues, muscles, all endothelial cells, as of joint-cavities, blood-vessels, the pleura and peritoneum, the spleen, kidneys and ureters, and the reproductive bodies. The epiblast now with its mesoblastic lining begins to form the Chorion, an embryonic intrauterine appendage; and the endoderm encloses the Archenteron or primitive gut. Before the end of the second week of gestation the heart is indicated as two tubes in the mesoderm, and the blood-vessels begin to be produced in the yolk-sac. About the twelfth day the mouth-pit shows, and the gut-tract is partly separated from the yolk-sac. The medullary plate of the nervous system is laid down about the fourteenth day, and the nasal area is observable. The maternal blood escapes into spaces about the embryo enclosed by masses of embryonic cells, which have not separated from one another, but which are known collectively as Syncytium.