Despite the radical importance of embryology in the development of organi- cism, developmental biology remains philosophically underexplored as a theoretical and empirical resource to clarify the nature of organisms. This paper discusses how embryology can help develop the organisational definition of the organism as a differentiated, function- ally integrated, and autonomous system. I distinguish two conceptions of development in the organisational tradition that yield two different conceptions of the organism: the life- history view claims that organisms can be considered as (...) such during their whole ontogeny; the constitutive view distinguishes two periods in the life history, a period of generation and a period of self-maintenance of a constitutive organisation. Arguing in favour of the con- stitutive view, it will be claimed that the organisational criteria for the definition of organ- ism (i.e., differentiation, functional integration, and autonomy) can only be applied to the developmental system when it has entered the period of self-maintenance of a constitutive organisation. under the light of current research in developmental biology, it is possible to make explicit how organisms come to be as organisms. To this end, I explore key onto- genetic events that help us clarify the core aspects of animal organisation and allow us to identify the developmental stage that marks the ontological transition between an organism in potency and an organism in actuality. The structure of this ontogenetic unfolding paral- lels the conceptual structure of the very notion of organism; the generation of the being of a particular organism parallels its definition. (shrink)