M S Khaparde
Articles written in Proceedings – Animal Sciences
Volume 87 Issue 6 June 1978 pp 91-104
The embryo of
Volume 87 Issue 9 September 1978 pp 179-194
The amnion expands to accommodate the growing foetus without undergoing any noticeable histological change. The yolk sac is large and forms an extensive chorio-vitelline placenta on the lateral sides of the uterus during early stages of gestation. However, due to the extension of the exocoelom and the expansion of the amniotic cavity the vascular splanchnopleure of the yolk sac is separated from the placenta and is invaginated towards the abembryonic trilaminar omphalopleure. The endodermal cells of the yolk sac hypertrophy as the yolk-sac wall gets vascularized, and the mesodermal cells enlarge only during the final stages of pregnancy. The allantoic vesicle, which is large during early stages of gestation, becomes progressively reduced and is finally lost during the final stages of pregnancy. The placenta, which is extensive during early stages, becomes restricted to a discoidal structure on the mesometrial side of the uterus. The definitive placenta is labyrinthine and vasomonochorial. Numerous free chorionic villi, which come into direct contact with the decidua at the margin of the placental disc, constitutes an accessory syndesmochorial placenta.