FIG. 1016. Diagrams illustrating the development of the foetal membranes of a Bird. A, early 

 stage in the formation of the amnion, sagittal section ; B, slightly later stage, transverse section ; 

 C, stage with completed amnion and commencing allantois ; D, stage in which the allantois has 

 begun to envelop the embryo and yolk-sac. The ectoderm is represented by a blue, the endoderm by 

 a red line ; the mesoderm is grey. all. allantois ; all', the same growing round the embryo and yolk- 

 sac ; am. amnion; am./., am./.' amniotic fold; an. anus; br. brain; cod. ccelome; cod', extra-em- 

 bryonic coelome ; hi. heart ; ms.ent. mesenteron ; mth. mouth ; ncli. nptochord ; sp. cd. spinal cord ; 

 sr. m. serous membrane ; umb. d. umbilical duct ; vt. m. vitelline membrane ; yk. yolk-sac. 



