The invention lies in the field of conifer tissue culture. It is especially directed to a method for production of large concentrations of embryonal cells particularly suitable for genetic modification. The cells are enriched in the sense that the ratio of embryonal to nonembryogenic cells is very significantly in crased over that attained by earlier methods. The method comprises first developing an embryonalsuspensor mass containing early stage proembryos from a suitable explant. These are multiplied in a maintenance medium and developed to late stage proembryos in a medium that preferably has an osmotic level raised about 30-50% over the initiation medium. The late stage proembryos are transferred to a new medium that again has the osmotic level raised about 35-55% over the proembryo development medium. After three to six transfers a large number of embryonal cells without suspensors will have formed. These may be used in any of the known methods for genetic alteration.
