In cartilaginous fishes, the middle portion of the tube develops as a shell gland. The first portion of this gland secretes the egg white, while the lower portion secretes a hard, horny, capsule to protect the developing egg. Below the shell gland is the ovisac, a distended region in which eggs are stored prior to laying. In ovoviviparous species, the egg remains within the ovisac until it hatches. Some cartilaginous fishes, however, are truly viviparous, giving birth to live young, and producing no egg shell. In these forms, the ovisac nurtures the developing embryo, often with the aid of vascular outgrowths similar to, but much simpler than, the mammalian placenta.