Comparison of axon guidance defects in <i>mid</i>, <i>slit</i> and <i>robo</i> mutants.

<p>Embryos from wild type (panels A–C), <i>mid H15</i> deficiency (panels D–F), <i>slit</i> (panels G–I) and <i>robo</i> (panels J, K) were stained with an antibody against Fas II. Arrow indicates projection from the pCC neuron as well as the medial tract (in older embryos) or its abnormal crossing at the midline; arrowheads in panel F indicate breaks, and star indicates groups of stalled longitudinal axon tracts. These <i>mid</i>-specific defects are not seen in either <i>slit</i> or <i>robo</i> mutant embryos. Anterior end is up, midline is marked by vertical lines. Note that to define age of the embryo we prefer “hours” of development (at room temperature) to the traditional “stages” of development. We think that this is particularly important for describing events at the molecular level. Assigning different “stages” was done based on gross morphological milestones during embryogenesis, and each stage can be 10 minutes short or 2–3 hours long. For defining age or development at the molecular level this approach is not meaningful and can be misleading.