[Сергей_Р]

var my_div = document.getElementsByTagName('div');

var addButton = function (e) {
    if (e.target.className === 'block') {
        var my_button = document.createElement("button"),
            id_element = document.getElementById("button_v"),
            text = document.createTextNode("Кнопка");

        my_button.appendChild(text);
        my_button.setAttribute('id', 'plus_v');
        my_button.classList.add('my_button');
        id_element.appendChild(my_button);

    }
    my_div[0].removeEventListener('mouseover', addButton); 
}


var addCell = function (e) {
    var div = document.createElement("div"), 
        cell = document.getElementById("cell"), 
        text_v = document.createTextNode("Cell V");

    if (e.target.id === 'plus_v') {
        div.classList.add('block_vertical'); 
        div.appendChild(text_v);
        cell.appendChild(div); 

   
    }
}

var buttons = document.getElementsByTagName('button');

for (var i = 0, len = buttons.length; i < len; i++) {
    buttons[i].addEventListener('click', addCell, false);
}