EMBRYOLOGY


Head-Tail Folding
Due to the rapid growth in the median plane of the brain, amniotic cavity, and somites, the embryo elongates, with its head and tail ends folding under.



At the cranial end, the head will be folded under, with a very prominent forebrain. Just cranial to it, on the ventral side, will lie the newly positioned primitive heart, pericardial cavity, septum transversum, and bucco-pharyngeal membrane.
Longitudinal section of an embryo about to undergo head-tail folding





During this fold the future body cavities of the intra-embryonic coelom will find their future locations and the foregut will form from the endoderm of the yolk sac, resulting in a reduced yolk sac.
At the tail end, the endoderm of the yolk sac is incorporated into the embryo to form the hindgut region. The connecting stalk is now attached ventrally, with the allantois jutting into the embryo.