Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/179

Rh and Germinal spot, lying upon the yellow yolk of the unlaid egg. Whatever view be taken of the relations of the eggs of the Vertebrata, the important point to be noticed is that the embryo of a fish, batrachian, reptile, bird, or mammal, including man at an early stage of life, is a guitar-shaped body (Figs. 177, 167), consisting of three membranes lying over one another, and narrowly bound together (Fig. 168); and if we were ignorant of the animal whose egg had been transformed into such a body, it would be very often impossible to say what would result from its development. These membranes are called blastodermic, or tissue germinating from the organs of the future animal growing in them.

The skin and central nervous system are developed in the External, or upper membrane; the osseous, muscular, vascular, reproductive, and urinary systems, the walls of the alimentary canal, and its appendages, are produced in the Middle membrane; while the epithelium, which lines the alimentary canal and its appendages, the lungs, liver, etc., is derived from the Internal or lower membrane.

In speaking of the Primitive trace, at page 127, we called attention to the furrow known as the Primitive groove. As development proceeds, this furrow deepens, and if the