page_title: Embryonic development
section_title: Formation of the germ layers
hierarchical_section_title: Embryonic development / Formation of the blastula / Formation of the germ layers
caption_attribution_description: English: Title: Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus Identifier: comparativeembry00nels (find matches) Year: 1953 (1950s) Authors: Nelsen, Olin E. (Olin Everett), b. 1898 Subjects: Vertebrates -- Embryology; Comparative embryology Publisher: New York, Blakiston Contributing Library: MBLWHOI Library Digitizing Sponsor: MBLWHOI Library View Book Page: Book Viewer About This Book: Catalog Entry View All Images: All Images From Book Click here to view book online to see this illustration in context in a browseable online version of this book. Text Appearing Before Image: PRE-CHORDAL .PLATE\ NEURAL PLATE ECTODERM iL ECTODERM YOLK SAC Text Appearing After Image: P R I M I T I V E NODE â PRIMITIVE P IT L A S T 0 PORE) PRIM I T IV E STREAK CHORION CHORIONIC VILLI PRIMITIVE "STREAK ALL A NT OE NTER I C DIVERTICULUM* BLOOD VESSEL Fig. 207. Three-germ-layer late gastrula of human embryo (pre-somite stage, after Heuser: Contrib. to Embryo). Carnegie Inst., Washington, Publ. 138, 23: 1932). (A) Dorsal view of embryonic disc, amnion removed. (B) Sagittal section through median area of (A). Observe elongated notochordal canal. (C-G) Transverse sections as mdicated. 432 Note About Images Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.
context_page_description: Embryonic development, also embryogenesis, is the process by which the embryo forms and develops. In mammals, the term refers chiefly to early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.
  Embryonic development starts with the fertilization of the egg cell by a sperm cell. Once fertilized, the ovum is referred to as a zygote, a single diploid cell. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo.
  Although embryogenesis occurs in both animal and plant development, this article addresses the common features among different animals, with some emphasis on the embryonic development of vertebrates and mammals.
context_section_description: The embryonic disk becomes oval and then pear-shaped, the wider end being directed forward. Near the narrow, posterior end, an opaque streak, called the primitive streak, makes its appearance and extends along the middle of the disk for about one-half of its length; at the anterior end of the streak there is a knob-like thickening termed the primitive node or knot, (known as Hensen's knot in birds). A shallow groove, the primitive groove, appears on the surface of the streak, and the anterior end of this groove communicates by means of an aperture, the blastopore, with the yolk sac. The primitive streak is produced by a thickening of the axial part of the ectoderm, the cells of which multiply, grow downward, and blend with those of the subjacent endoderm. From the sides of the primitive streak a third layer of cells, the mesoderm, extends laterally between the ectoderm and endoderm; the caudal end of the primitive streak forms the cloacal membrane. The blastoderm now consists of three layers, named from without inward: ectoderm, mesoderm, and endoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that implantation of the embryo in the uterus of the mother occurs.
caption: Comparative vertebrate embryology.
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