A Textbook in General Zoology

A Textbook in General Zoology

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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. 1906 edition. Excerpt: ...and to make another cavity with a wide opening to the outside (Fig. 105, K, F). This is the yastrula (little stomach) stage. It is appropriately called so, because at this time the embryo first begins to take food in the egg-capsule; the food is digested in the cavity formed from the outside. In the gastrula it is possible to distinguish two layers of cells, both of which we know came originally Fio. 105. Early Stages of Development of Earthworm. Much enlarged A, blastula, surface view. //, hlastnla, section: s.c, blastula cavity; m, first cell of mesoderm. (', blastula, later stage. 1), blastula, flattening of future endndermal cells. K, gastrula, in side view: ec, ectoderm; en, endoderm;.i s. line of section for F, gastrula, cross section: ar, arehenteron (From Sedgwick and Wilson's General Biology) from a single layer; these two layers are called ectoderm (Fig. 105, H, ee) and endoderm (Fig. 105, E, en). While the embryo is still in the blastula stage a single cell (Fig. 105, /?, To) can be identified as one which later develops into a row of cells on either side, and finally into a mass of tissue on either side, to which we give the name mesoderm (Fig. 105, F). Many animals pass through these stages; hence the use of the terms ectoderm, endoderm, and mesoderm, as the primary germ-layers, has very great significance. Already we can note many changes from the simple blastula condition to the advanced gastrula. In discussing these and subsequent changes, we shall use the word " differentiation," which means literally "a becoming different." Differentiation increases after the gastrula stage, and the embryo becomes longer and larger, as organ after organ makes its appearance out of one or the other of the three germ-layers....show more

Product details

  • Paperback | 114 pages
  • 189 x 246 x 6mm | 218g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236590457
  • 9781236590459