Carnegie Stage 5a-1

Introduction

Stage 5a specimens have a postfertilization age of 7 to 8 days and are characterized by a trophoblast that is still mainly solid. The greatest diameter of the conceptus is less than 0.5 mm. The blastocystic cavity is flattened probably because of its collapse during implantation. Mesoblast formation begins along the inner side of the trophoblast. The embryonic disc is approximately 0.1 mm in diameter and is composed of two layers, a thick layer called the epiblast and a thin layer called the hypoblast. The amniotic cavity is apparent for the first time and is formed by the curved epiblast of the embryonic disc.