Staging of middle and late embryonic development in the medicinal leech, Hirudo medicinalis

We present a description of the last half of embryonic development in the E uropean medicinal leech, Hirudo medicinalis, based entirely on externally v isible morphological features, and establish reliably observable stages dur ing that development. Embryogenesis, from the time fertilized eggs are depo sited in an eggcase (called a cocoon) to the emergence of juveniles from th e cocoon, takes approximately 4 weeks at roam temperature. The stages descr ibed in this paper extend from the completion of segmentation to the appear ance of the final bands of pigmentation. Developmental stages are expressed as percentages of total embryonic developmental time. This staging table w as constructed for embryos kept at 20 degrees C. In addition, the developme nt of animals kept at 17 degrees C or at 24 degrees C was compared with tho se held at 20 degrees C. Development proceeds more quickly at higher temper atures. Because development in embryos held at higher or lower temperatures was linearly related to the stages determined for embryos held at 20 degre es C, the rate of development at any intermediate temperature can be predic ted from the staging table at 20 degrees C by simple multiplication. J. Com p. Neurol. 402:155-167, 1998. (C) 1998 Wiley-Liss, Inc.