Science for All. Ed. by R. Brown (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1883 Excerpt: ...im) External I--)er of Shell Membrane; (I) Shell. concentric with the external layer. Resting on the yolk, immediately beneath the yolk-membrane, will be seen a small, whitish, disc-like body, about one-eighth of an inch across. If this be examined with a lens, it will be seen to exhibit two more or less well-defined parts--an outer white ring, and an inner transparent circular space, in which dots of white are usually seen (Fig. 2). This disc is the so-called blastoderm.t From it, and from it alone, the future chick will be developed, the remainder of the yolk serving only as nutriment for the chick until hatched. The central clear space is called the pellucid area, the outer white ring the opaque area. It is in the pellucid area that the chick is developed, the opaque area giving rise to certain temporary structures, which serve a purpose ending with the hatching of the egg. As shown in the diagram (Fig. 1), the blastoderm rests upon the top of the Latin, vittllut, yolk. t Greek, Uattoi, a germ; ami i/c.-mu, a akin. flask-shaped mass of white yolk, between which and its lower side is a small cavity filled with clear fluid, in which a few cells may be seen floating. The blastoderm itself consists at this period of two layers of cells, and the upper layer, extending beyond the edges of the lower one, rests directly upon the white yolk, and gives rise to the opaque area. Fig. 2.--Blastoderm u __, seen from Above. Ihese are the structures (auoraviueArea;i/aimiurin present in the hen's egg when laid; but in order to thoroughly comprehend their history and relative importance we must go back to a much earlier stage, and trace their formation in the body of the hen. As every one knows who has ever prepared a fowl for cooking, there is always found in the bo...