Changes in the Density of Sodium Currents in the Membranes of Spinal Cord Nerve Cells During Early Stages in the Development of the Chick Embryo [Izmenenie Plotnosti Natrievykh Tokov v Membranakh Nervnykh Kletok Spinnogo Mozga Na Rannikh Étapakh Razvitiia Kurinogo Émbriona.]