The primer is an extensively crosslinked hypertext document. You can take a "tour" of the site by clicking on the first link at the base of each page you visit. This route will take you from this page through the text items and diagrams in the table of contents, and then to the first image in the first Photoshop image gallery. The tour then takes you down each column of images in turn (left to right).
There are many other possible paths through the document. Each page is linked to several other pages to which one might want to move. For example, three focal planes of SNb in the same segments are linked together, so that you can move between them as if you were moving between focal planes on the microscope. Each page is also linked to a diagram of the muscles and motor nerves, high-power views of nerves are linked to brightfield images of whole embryos at the same stages, and pages are linked along a developmental timeline. Note that if you are flipping back and forth between the diagrams and an image, you can use the Back key to return to the image from the diagrams. (Each image has a link to the diagrams, but the diagrams do not have a link to every image.) To see the full image for most of the views, expand the window to fill the full width of the monitor. The images are best viewed on a 15" or larger monitor, because a 13" monitor is only 480 pixels in width and many of the images are 600 pixels wide.
A recent review of the fly motor axon system is: Keshishian et al. (1996) Ann. Rev. Neur. 19, 545-575. The 1993 Development of Drosophila melanogaster (Cold Spring Harbor Press) is a useful reference work to consult while perusing the primer, as it contains detailed chapters (with diagrams) on the development of the mesoderm (M. Bate), CNS (C. S. Goodman and C. Q. Doe) and peripheral nervous system (Y-N. Jan and L-Y. Jan). Finally, the green Campos-Ortega and Hartenstein Embryonic development of Drosophila book (Springer-Verlag) is a useful reference for precise staging of embryos.
The motor neurons that innervate the embryonic muscles have been identified by the Keshishian and Whitington groups, and these papers are referenced in the review cited above. This primer, unfortunately, does not contain images of dye-filled neurons or a map of the cell body positions of the motor neurons. This would be an extremely useful addition to this resource, however, and we would welcome the contribution of images of dye-filled neurons and/or links to other sites containing such images.
The primer uses primarily images from "typical" embryos, rather than showing flattened preparations that might be used for publication because they show a nerve in a single focal plane. For comparison of a mutant phenotype observed in dissected embryos to the wild-type embryos shown here, one can use the CNS and brightfield photos as guides for staging your embryos, then flip through the focal planes for the nerve of interest and compare the patterns observed to your own embryos. The muscle fibers (many of which are labeled in the images) provide the most useful reference points for knowing which focal plane you are in. Note also that when you click the links to flip between focal planes, you can use the New Window option (hold down the mouse button while positioned on the link) to simultaneously display two or more images.
axon phenotypes are scored in late embryos. These are
normally fixed, cracked with methanol, stained with
antibodies such as MAb 1D4 using HRP immunohistochemistry,
glycerol-cleared, and dissected. The full set of motor axon
branches does not develop until stage 17, shortly before the
embryos can no longer be stained with antibodies. For
accurately scoring phenotypes, late stage 16 or early stage
17 embryos should be used. The pattern in the early 17
embryo serves as the standard, and most of the images in
this collection are from embryos at this stage. Most studies
focus on the abdominal segments, because A2-A7 have
essentially identical patterns and can thus be scored
together (giving you up to 12 hemisegments/embryo). A1 has
minor differences, and the thoracic segments have some major
differences. A8 and A9 also have unique patterns. Only
segments A2-A7 are discussed here.