Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/177

Rh egg in a slightly more advanced stage be now examined from a horizontal point of view, there will be seen a light oval space, the area pellucida, surrounded by a dark space, the area opaca (Fig. 166); within the area pellucida will be noticed an oval body, the Primitive trace, so called from indicating the position of the embryo, the furrow in the Primitive trace being known as the Primitive groove. A little later the Primitive trace and area pellucida become guitar-shaped (Fig. 167), and if a longitudinal section of the egg be examined (Fig. 168) it will be seen to consist of the External and Internal blastodermic membranes, and a third membrane lying between these two. The partial fusion of these membranes makes the Primitive trace, While these three membranes are consolidating into the Primitive trace, the Middle membrane splits into two layers: the Upper terminates in the External blastodermic membrane, the Lower grows gradually around the Internal blastodermic membrane, finally inclosing it. The embryo at this period is a guitar-shaped body (Fig. 177), consisting simply of three membranes lying over one another, narrowly bound together. The question may be asked by some of our readers. What relation does so minute a structure as the egg of a mammal bear to that of a bird? Does the development of a rabbit resemble that of the chick? The egg of a chicken (Fig. 174), as all the world knows, is composed of a shell inclosing a semi-liquid substance, in which is suspended a yolk. If a freshly-laid egg be carefully examined, however, supposing the conditions to have been favorable to development, there will be found lying on the top of the yolk a delicate sheath (Fig. 174, b), which is composed of two membranes; while the yolk itself, if laid open, exhibits in its interior a whitish body (Fig. 174, a), which, narrowing into a thread, runs upwards towards the membrane composing the sheath. This whitish substance is called the white yolk, as