Ovoviviparous - Rantaisei

In animal development, this refers to a pattern in which internally fertilized eggs hatch inside the mother's body, and after a certain amount of growth, they are released outside the mother's body without having any close tissue connection to the mother. In other words, it can be said to be somewhere between oviparity and viviparity. In ovoviviparity, the degree of dependence on the mother for nutrition varies, from those that are almost completely independent of the mother's nutrition, such as pond snails and pit vipers, to those that obtain nutrition from the mother's ovaries through the body epithelium and gill slits, such as the sea bitterling, and even those that connect to the mother's gonads through a yolk sac and obtain nutrition directly from the mother, such as many cartilaginous fishes such as sharks and rays. In particular, the starry hound shark has developed a structure similar to the placenta, and the difference from true viviparity becomes unclear.

[Amanami Amemiya]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

動物の発生に際し、体内受精した卵が母体内で孵化(ふか)後、母体と密接な組織的結合をもたないまま、ある程度成長したのちに母体外に出る様式をいう。すなわち卵生と胎生の間に位置するといえる。卵胎生においては、栄養上で母体に依存する程度はさまざまで、タニシやマムシのようにほとんど母体の栄養に依存しないものから、ウミタナゴのように体上皮、鰓孔(さいこう)を経て母体の卵巣から栄養をとるもの、さらにサメ、エイなどの軟骨魚類の多くのように、卵黄嚢(のう)により母体の生殖巣と連絡して、母体から直接に栄養を摂取するものまである。なかでも、ホシザメでは胎盤に近い構造が発達し、真の胎生との相違は明らかでなくなる。

［雨宮昭南］

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