36 HISTOLOGY 



derm, and the other (the splanchnic layer) to the entoderm. Between 

 them is a cavity, known as the body cavity or coslom, which in the adult 

 becomes subdivided into the peritoneal, pleural, and pericardial cavities. 

 The ectoderm and the somatic mesoderm together form the body wall or 

 somatopleure; the entoderm and the splanchnic mesoderm together form 

 the intestinal wall or splanchnopleure. 



Reviewing the preceding paragraphs it is seen that the fertilized ovum, 

 through segmentation, forms a morula, which later becomes a vesicle 

 composed of three germ layers, the outer or ectoderm, inner or entoderm, 

 and middle or mesoderm. By the folding of these layers the body as a 

 whole acquires its form; and by their growth and differentiation all the 

 organs and tissues are produced, together with the fetal membranes which 

 surround the embryo. Omitting for the present all reference to the mem- 

 branes, the fundamental changes which the germ layers undergo may be 

 briefly considered, as follows: 



Ectoderm. A portion of the ectoderm forms a layer of cells covering 

 the body of the embryo. In the adult this becomes the outer layer of 

 the skin, or the epidermis, and from it, hairs, nails and the mammary, 

 sebaceous and sweat glands develop. It is reflected under the eyelids 

 and over the front of the eye, and forms the lachrymal glands. It etxends 

 into the external auditory opening and there forms the ceruminous glands; 

 and into the nasal, oral, anal and urogenital apertures. Within the 

 mouth it forms the salivary glands, the enamel of the teeth, and the cells 

 associated with the sense of taste. Thus it extends far back toward 

 the pharynx, and dorsally, in its deepest part, it produces the anterior 

 lobe of the hypophysis, which will be described in a later chapter. In 

 the nose it also extends far inward, so that it lines the accessory cavities 

 which push out from the nasal cavity into certain bones of the head, and 

 it forms the olfactory cells. An inpocketing of the ectoderm produces 

 the lining of the deep portion of the ear, including the auditory cells, and, 

 as will be seen, the ectoderm gives rise to the lens and retina of the eye. 

 Thus the ectoderm not only forms the outer covering of the body, with 

 extensions into the several apertures, but it produces various sensory 

 cells which are stimulated from external sources. 



The second great derivative of the ectoderm is the nervous system. 

 It arises in young embryos as the medullary groove. This is a longitudinal 

 groove or furrow, situated in front of the primitive knot and appearing 

 in cross section as a median dorsal depression (Fig. 25, G and H). Later 

 the groove becomes a tube by the coalescence of its dorsal edges, which 

 are about to unite in Fig. 25, H. The tube then becomes completely 

 separated from the epidermal layer of ectoderm, as in Fig. 29. 



The closure of the medullary groove to form a tube begins near the 

 anterior end of the embryo and proceeds backward. Thus for a time the 



