Answer:
interstitial fluid; ductless; ducts
Explanation:
Endocrine glands secrete their hormones directly into the interstitial fluid (blood) rather than using via a specified duct. Major glands of endocrine system include pituitary gland, pancreas, etc. They are the glands which do not use any duct, so thats why they are also known as ductless glands.
Exocrine glands secrete their products onto epithelial surface by the way of a duct. Major glands of exocrine glands include salivary gland, mammary gland, etc.