In the region of the head it exhibits on each side a slight bulging outwards, the rudiment of the first visceral cleft.
The first traces of the urinary system become visible at about the time of the appearance of the third visceral cleft.
The figure shews The mode of formation of a visceral cleft without any involution of the external skin.
The first visceral cleft (1vc) and eye (op) are just commencing to be formed, and the cranial flexure has just appeared.
visceral cleft n.
A cleft between two branchial arches in the embryo.