ORGANS IN THE EMBRYO OF THE FOWL. IOI 



At the time of formation of the tail-bud a very shallow pocket 

 forms behind it. This owes its origin to the elevation of lateral 

 folds of the somatopleure and progressive fusion beginning at the 

 posterior angle of the ectamnion. The floor of the pocket in- 

 cludes a thick posterior prolongation of the allantoic mesoblast 

 which furnishes a firm floor to the pocket and thus determines 

 the form of the folds. 



2. The Amniotic Folds.- -The subsequent development in- 

 cludes the elevation and fusion of the anterior and posterior lat- 

 eral folds. The final closure takes place opposite the buds of the 

 hind limbs. The order of events in these processes is as follows : 



1. The growth of the amniogenous somatopleure behind the 

 head-fold and in front of the tail -fold. 



2. The uprising of the amniotic folds, and their growth in a 

 definite direction around the embryo. 



3. The fusion of the right and left folds along the line of the 

 ectamnion in such a way that the external limbs unite to form 

 the chorion, and the internal to form the amnion. 



Study of the morphology of these processes suggests the fol- 

 lowing physiological conclusions : 



1. The growth of the amniogenous somatopleure may be a 

 result of the traction exerted in it by the progressive fusion of the 

 folds already formed in front and behind. 



2. The uprising of the lateral folds is determined by the head- 

 and tail-folds, the progressive fusion of the right and left ectam- 

 nion dragging the amniogenous somatopleure into place. 



It remains to test these conclusions by experiments, but before 

 proceeding to a description of these, I wish to describe the influ- 

 ence of the rotation of the embryo on the amniogenous somatopleure. 



Practically all of the somatopleure of the pellucid area is amni- 

 ogenous with the exception, naturally, of that part internal to 

 the limiting sulci that forms the body-wall. What effect has 

 the turning of the embryo on its left side on the amnrogenous 

 somatopleure ? We will suppose that the latter is primitively of 

 equal width on both sides ; we will furthermore assume that the 

 somatopleure cannot be drawn in from the vascular area, because 

 it is here attached to the splanchnopleure. (The fusion of the 

 somatopleure and splanchnopleure at the margin of the pellucid 



