THE F(ETAL MEMBRANES OF REPTILES AND BIRDS. 



209 



covers the anterior part of the head like a thin transparent veil, and 

 is therefore called the cephalic sheath. 



In like manner, but at a somewhat later stage, there arise at the 

 tail-end and at both sides of the embryo the posterior and lateral 

 folds of the amnion. Tlie posterior fold is still very inconspicuous even 



at the time when the head is covered with the veil-like pellicle 

 (Plate I., fig. 11 haj). It enlarges slowly, and under the name of 

 caudal sheath covers over the posterior end of the body (fig. 126 am). 

 The lateral folds of the amnion are elevated externally to the lateral 

 marginal grooves (fig. 127 om), and project in the opposite direction 

 from those lateral folds by the bending in of which the lateral and 

 ventral walls of the embryo are produced. By this means the rim 



14 



