ORGANS IN THE EMBRYO OF THE FOWL. 



body-wall (section 138, Fig. 10) and extends to section 167, where 

 it suddenly ceases, a distance of about 0.5 mm. The location is 

 indicated by x on Fig. 9, and Fig. 10 shows it in section. 



The formation of this fold is not induced by the ectamnion be- 

 cause the line of the latter (Fig. 10, r.e.a.} may be recognized 

 some distance lateral to the fold, through it is very slightly 

 developed. The fold in question is immediately back of the 

 heart on the right side of the body. It is not, in my opinion, a 

 true amniotic fold, but belongs to the category of normal sec- 

 ondary folds of the amniogenous somatopleure produced by the 

 turning of the embryo, with which it agrees precisely in position 

 and appearance. This conclusion is reinforced by the following 

 consideration : in this embryo the roots of the vitelline veins are 

 prolonged forward to an abnormal extent, and the right vein is 

 fused to the somatopleure lateral to the fold (Fig. 10). As the 



l.a.J. 



r.e.a. 



FIG. IO. Section through the embryo of experiment 57 along the line A-B of Fig. 

 9. l.a.f., left amniotic rudiment ; r.e.a., ectamnion of the right side ; s.f. , secondary 

 fold of amnion on the right side ; v.?>., vitelline veins. 



embryo turns, therefore, the somatopleure between the vitelline 

 vein and the body-wall must be folded to the extent that the 

 turning approximates the body-wall to the vein, because the 

 fusion prevents the somatopleure from being pushed peripherally. 

 As already said, therefore, this is not a true amniotic fold. 



The prevention of the formation of the head-fold, by destruc- 

 tion of the rudiment of one side, operates to prevent the normal 

 elevation of the amniotic fold on the opposite side ; and thus it is 

 experimentally demonstrated that the cooperation of right and 

 left folds is necessary for the normal mode and direction of 



