ORGANS IN THE EMBRYO OF THE FOWL. 113 



of the tail. But beneath the latter they form a closed pocket, 

 the usual tail-fold. 



Over the entire region, extending from about the posterior 

 edge of the fore-limb to the beginning of the hind-limb, there are 

 no folds in the amniogenous somatopleure. This would indicate 

 that the normal rapid growth of this region is progressively in- 

 duced under normal conditions by the extension of the lateral 

 angles of the head-fold backwards. The folds shown in the 

 figure are only from about 354430 and may be explained as 

 remnants of the original head-fold, the postero-lateral prolonga- 

 tions of which were probably not entirely removed by the oper- 

 ation. These folds have not, however, united over the embryo 

 nor have they induced formation of folds behind them. The 



FIG. 13. Experiment 36. Section of embryo along the line A-B, Fig. 12. The 

 irregular and incomplete amniotic folds are well shown, e.a., ectamnion of the right 

 side. 



reason for this is clear when we consider that the normal process 

 involves continuous traction on the somatopleure back of the 

 advancing folds, for the latter are continually fusing along the 

 dorsal line with those of the opposite side and thus are con- 

 stantly, so to speak, gathering in the slack, and causing tension. 

 In the drawing of the entire embryo, the left side is upper- 

 most, but at the time of the operation the right side was up. 

 Evidently the embryo was turned over after removal of the blas- 

 toderm in the process of preparation. This explains why in the 

 section the lower amniotic fold has the usual appearance of the 

 upper fold. The ectamnion is visible only on the left side of the 

 drawing; on the right side no trace of it could be found, except 

 in the region of the tail-fold. 



