The mode of secretion of sebaceous gland is
Sebaceous  Glands 
Sebaceous glands, producing an oily secretion, are widely  distributed all over the dermis of the skin, except for the palms and soles.
Sebaceous glands are holocrine in nature, i.e. they produce their secretion by complete fatty degeneration of the central cells of the alveolus, which are then replaced by the proliferating peripheral  cells. The secretion is under hormonal control, especially the androgens. The oily secretion of sebaceous glands is called sebum.
Reference: BD Chaurasia’s Handbook of GENERAL ANATOMY Fourth  Edition page no 180
Merocrine
Holocrine
Apocrine
Eccrine