The theory Lysander postulates would seem reasonable. When the gametes of an oak tree are joined, the product that forms is not generally called a "tree" until such time as it has taken root and started to grow. Until that time, it would be called an "acorn".
BTW, perhaps a biologist can answer for me a question: does a pre-implantation zygote metabolize sugar or some other energy source from its environment, or is all of the energy required for pre-implantation growth contained within the original zygote?
The acorn is an oak acorn. It is the species of it's parents, although it does not demostrate all the characteristics of the seedling, sapling or full grown tree stages of development of the oak. It is not necessary for members of the species to demonstrate all of the characteristics of the species at any one time or even at any time in the life span in order to qualify as a member of the species. For most purposes it is only necessary to be of the species by parentage (usually sufficeint for classification) and/or to demonstrate the appropriate characteristics of that stage.
The embryo utilizes nutrients from the environment in the fallopian tube and in the uterus, even before gastrulation and implantation. (The zygote is appropriate nomenclature only at the one cell stage of the embryo.)