II.] 



THE AMNION. 

 FIG. 11. 



45 



DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POS- 

 TERIOR END OF AN EMBRYO BIRD, AT THE TIME OF THE 

 FORMATION OF THE ALLANTOIS. 



ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric 

 canal ; liy. hypoblast ; p.a.g. postanal gut ; pr. remains of 

 primitive streak folded in on the ventral side ; al. allantois ; 

 me. mesoblast; an. point where anus will be formed; p.c. 

 perivisceral cavity ; am. amnion ; so. somatopleure ; sp. 

 splanchnopleure. 



Each fold, of course, necessarily consists of two 

 limbs, both limbs consisting of epiblast and a very thin 

 layer of mesoblast ; but in one limb the epiblast looks 

 towards the embryo, while in the other it looks away 

 from it. The space between the two limbs of the fold, 

 as can easily be seen in Figs. 9 and 11, is really part 

 of the space between the somatopleure and splanch- 

 nopleure ; it is therefore continuous with the general 

 space, part of which afterwards becomes the pleuro- 

 peritoneal cavity of the body, shaded with dots in 

 figure 9 and marked (p p). It is thus possible 'to 

 pass from the cavity between the two limbs of each 



