Attempts to disprove A.N. Leontiev's finding known as the "memorization development parallelogram" are shown as groundless. As nobody (including the author) has obtained direct confirmation of L.S. Vygotsky and A.N. Leontiev's hypothesis of "growing in", it is suggested that two notions should become descriminated: "the memorization development parallelogram" and "the principle of the parallelogram". The latter is described as a manifestation of L.S. Vygotsky's "principal genetic law of cultural development" and the soft spot of the cultural historical theory. It is stated that, although A.N. Leontiev has transformed L.S. Vygotsky's ideas, the law in its substance (from the outer through internalization to the inner) has remained unchanged. The main idea of the article is "from "the principle of the parallelogram" to the activity theory".