FCETAL MEMBRANES. 



101 



embryo they break through at the line of contact, thus leaving the outer layers 

 of the folds continuous and the inner layers continuous, with the extraembryonic 

 body cavity continuous between the outer and inner layers. 



t. am. f. 



pr. g. 



ent. 



mes. b.c. al. a.m. eg. ta. 



FiG. 92 — Medial section of caudal end of chick embryo (at end of second 

 day of incubation). Duval, 

 al., Beginning of allantoic evagination; A.m., anal membrane; b.c, extraembryonic body cavity 

 e.g., caudal gut; ect., ectoderm; ent., entoderm; mes., mesoderm; mes.*, parietal mesoderm 1 

 mes.', visceral mesoderm; to. to., neural tube; pr. g., primitive gut; t. am. !., tail amniotic 

 told; ta., tail. 



The result of the development of the amniotic folds is :— 

 1. That the embryo is completely enclosed dorsally and laterally by a cavity, 

 the amniotic cavity, which is lined by ectoderm continuous with the ectoderm— 



Dorsal amniotic 

 suture 



Primitive 

 streak 



Fig. 93. — Dorsal view of embryo of albatross, showing amnion covering cephalic 



end of embryo. Schauinsland. 



x, Portion of blastoderm containing no mesoderm. 



later epidermis — of the embryo, the ectoderm lining the cavity and the overlying 

 parietal mesoderm together constituting the amnion (Fig. 96). 



2. That the outer parts of the amniotic folds become completely separated 



