Stage 13 embryos have 30 or more somites, although by this stage it is getting increasingly difficult to distinguish individual somites and make accurate counts. Stage 13 is the first stage in which all four limb bubs can be recognized. The upper limb bud , which may be just visible in some stage 12 embryos, is now a definitive elevated ridge, and the start of stage 13 is marked by the first appearance of the lower limb buds.
Other features of this stage are the first appearance of the lens disc and the closure of the otic vesicle (pit). Internally, the lung buds appear, a distinct trachea begins to develop and the septum primum and foramen primum appear in the heart.
Most stage 13 embryos have a greatest length of 4 to 5 mm, although they can range from 3 to 6 mm. If greatest length is taken as the only measure of embryo growth it would appear that growth slows between stages 11 and 13, however, it should be noted that during these stages the embryos are also growing in the other two dimensions and increasing in bulk not just length.
Stage 13 embryos have an estimated postfertilization age of about 32 days