Over the next days (21 to 23 days) the upper lip is formed and eyelids begin to develop. Earlobes are formed. At the end of the legs, toes can be distinguished as a fan of dark matter against a lighter background. The genitalia are developed further. Internally, the skeleton and the muscles differentiate further and the ribs are formed, amongst others. The skeleton that is formed, still consists of cartilage. The face develops, tongue and palate are formed. Thyroid gland, parathyroid gland and heart develop, and an organ that most people at best only know as "sweetbread" from haute cuisine. The common biomedical term for sweetbread is thymus. This is situated in the chest cavity between the breast bone and the heart. The thymus has a central function in the immune system and is responsible for the production of so-called T-cells. If the thymus is absent in newborn animals, they produce hardly any antibodies at all and are therefore very susceptible to infections. But if the thymus is experimentally removed from animals (mice) when they are a few days old, these problems will not occur so much: the lymphocytes that are produced in the thymus and move on to the spleen and lymph glands, probably have already passed on the necessary information about what is foreign to spleen and lymph gland. The embryo now measures 13mm to 30mm.