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It is important to note that by 48 hours of incubation, the shape of the chick embryo has become more complex than its earlier straight line configuration. From 33 hours to 48 hours the embryo has undergone rapid growth of its anterior end and has developed both a cranial flexure and a dextral torsion of its anterior end. A flexure is a ventral bend in the cranio-caudal axis of the embryo and the cranial flexure is a bend at the midbrain region. The anterior portion of the embryo has undergone a dextral torsion which means that the anterior half of the embryo has twisted about its cranio-caudal axis so that the front half of the embryo is laying on its left side with respect to the yolk. The posterior half of the embryo is still laying dorso-ventrally with respect to the yolk. Serial cross sectioning of the 48 hour chick begins at the midbrain region rather than at the forebrain region due to the cranial flexure of the embryo.
The photographic frame contains a photomicrographic image of a cross section of a 48 hour chick embryo taken from a set of 480 serial cross sections. The photographs are numbered one to 45 out of a possible 45. The "Next Section" button below the photograph allows you to click and move to the following photograph. Clicking on the "Zoom" button will open a new window with the section at a higher magnification.
The cross sectional photographs are presented with an inward-looking view of the sectioned embryo. To understand this perspective, imagine yourself standing in front of the whole embryo with the head closest to you, the yolk (and the embryo's left side) on the right hand side, and the tail farthest away from you. Imagine cutting a section through the embryo, discarding the anterior portion, and then looking into the sectioned embryo. For the anterior half of the embryo, the embryo's left side will appear on the right hand side of the photograph and correspondingly the embryo's right side will appear on the left hand side of the photograph. The more caudal portion of the brain and spinal cord will appear uppermost, and the more cephalic portion will appear lowermost. For the posterior half of the embryo, the embryo's dorsal surface will appear on the left hand side of the photograph and the embryo's ventral surface will appear on the right hand side of the photograph. The embryo's left side will appear uppermost and the embryo's right side will appear lowermost.
The larger frame on the right hand side of the screen is the text frame. This frame has a scroll bar on the right hand side of the frame that allows you to move text upward and downward. The text consists of descriptive paragraphs that begin with a title that gives the section number out of the total number of 480 sections that comprise the embryo. A listing of the major visible features occurring at this particular level of sectioning follows the section number. Clicking on the title will bring the corresponding photograph into the photographic screen.
The orientation frame is located on the lower right hand side of the screen. This frame contains a thumbnail size photograph of the dextrodorsal view of a whole 48 hour chick embryo. A vertical line indicates the level of sectioning of each cross sectional photograph. A button list allows you to click and move to the next section, a previous section, the ZOO*2100 (Developmental Biology) index, or the main (Cell and Developmental Biology) index page.
Note that the images are designed to display optimally with a screen resolution
of 600 X 800 pixels. You may wish to adjust your video display. (Hint: Minimizing
some of the navigation bars in your browser will also increase the size of your
viewing screen and will also make the viewing of the serial section frames more
convenient.)
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Send comments to:
Sandra K. Ackerley (),
Department of Zoology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.
Page revised May 30, 2000