Fig. 1G4. — Diagram illustrating the dcvcloinncnt of {(vtal mcml)i'ancs in 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 liy blue, the eudoderm by red, and the 

 mesoderm by gray. <(//, allanlois; all', the same growing round the embryo and 

 yolk-,sac; am., amnion; diii. /, nm. /', amniotic fold; an, anus; hr, brain; cod, coeloni; 

 cod', extra-iMiibi yoiiic cailoin; //, hcirt; in.s.ciil, mesenteron; null, mouth; vc)i, 

 notoclioril; .■;/<. nl, spinal chord; .^r. in', serous membrane; iiinh. d, umbilical duct; 

 ;,'(, in, vitclliiie nii'niL)rauc; ///i", >'olk-sa,c. (From Parker and HaswcU.) 

 320 



