The elementary organs are typical with regard to their position and with regard to their histological properties. In many cases they are of a very clearly different histological type, as for instance, the cells of the three so-called germ-layers; and in other cases, though apparently almost identical histologically, they can be proved to be different by their different power of resisting injuries or by other means. But there are not as many different types of histological structure as there are typically placed organs: on the contrary there are many elementary organs of the same type in different typical parts of the organism, as all of you know to be the case with nerves and muscles. It will not be without importance for our future theory of development, carefully to notice this fact, that specialisation in the *position* of embryonic parts is more strict than in their histology. But elementary organs are not only typical in position and histology, they are typical also with regard to their form and their relative size. It agrees with what has been said about histology being independent of typical position, that there may be a number of organs in an embryonic stage, all in their most typical positions, which though all possessing the same histology, may have different forms or different sizes or both: the single bones of the skeleton of vertebrates or of adult echinoderms are the very best instances of this most important feature of organogenesis. If we look back from elementary organs to elementary processes, the specialisation of the size of those organs may also be said to be the consequence of a typical duration of the elementary morphogenetic process leading to them.[7] [7] The phrase “*ceteris paribus*” has to be added of course, as the duration of each single elementary morphogenetic process is liable to vary with the temperature and many other conditions of the medium. I hardly need to say, that the histology, form, and size of elementary organs are equally an expression of their present or future physiological function. At least they prepare for this function by a specific sort of metabolism which sets in very early. The whole sequence of individual morphogenesis has been divided by some embryologists into two different periods; there is a first period, during which the foundations of the organisation of the “type” are laid down, and a second period, during which the histo-physiological specifications are modelled out (von Baer, Götte, Roux). Such a discrimination is certainly justified, if not taken too strictly; but its practical application would encounter certain difficulties in many larval forms, and also, of course, in all plants.