Studying Vygotskij's ideas from his earlier work such as "The Psychology of Art" to his later work ("Thinking and Speech") is a fascinating experience of immersion in a world of ideas. The evolution of some of these ideas can be followed throughout his work (like the notion of "zone of proximal development", or 'sign"); others remain notions expressed in common language not yet transformed into academic concepts (like activity, leading activity, play). It was the work of his colleagues and followers to develop these ideas further into a coherent theory of human development. Leont"ev, for example, elaborated the notion of activity (dejatel"nost") and leading activity (veduščaja dejatel"nost") into an activity theory; El"konin has developed the notion of play and leading activity into a theory of ontogenetic development; Božovic worked on the notions of personality and the social situation of development; Zaporožec contributed significantly to the elaboration of the theories of movement and perception, and so on. The evolution of Vygotskij's ideas has been described in a number of accomplished works (Kozulin 1990; van der Veer and Valsiner 1991; Wertsch 1985; see for a brief historical summary regarding early childhood education: Veraksa and van Oers 2011). Diverse readings of Vygotskij still exist (see for example van Oers 2011).