DEVELOPMENT OF EMBRYO OF FOWL. 39 



the embryo was naked and there was slight ectopia of the intes- 

 tine (Fig. 2, C\ 



Figures 2, A, B and C show this embryo from the two sides 

 and from behind. The last figure shows especially well the 

 attachment of the amnion and the ectopia of the intestine ex- 

 ternal to this. The hind-end of the embryo back of the vitelline 

 vessels is wanting. On the other hand, all parts in front of this 

 are normally developed. 



On each side, at the hind-end of the Wolffian ridge, rudiments 

 of the hind limbs are present as prominent conical stumps. From 

 this it would appear at first sight that there has been some regen- 

 eration. However the question can be decided only by ascer- 

 taining the numerical value of the segments concerned in the 

 formation of these stumps. The embryo was cut into sagittal 

 sections for this purpose. 



The enumeration of the post-brachial nerves is the same on 

 both sides, seven pairs being present. The stumps are innervated 

 only by the seventh on each side (Fig. 3). In the normal chick, 

 the nerves innervating the leg are the seventh to the thirteenth 

 postbrachial. Thus it would appear that only one leg somite on 

 each side was uninjured by the operation, and this is the only 

 one that has contributed to the formation of the rudimentary leg. 

 None of the six somites between arm and leg has undergone 

 any alteration of its normal numerical value. This result must 

 therefore be interpreted in the sense of normal self-differentiation 

 of the somites concerned. 



Structure of the Leg-rudiments. But, even though the defi- 

 ciency of leg somites has not stimulated their immediate neigh- 

 bors in front to any act of supererogation, it might be that the 

 only leg somite remaining has produced more than its wont. In 

 this connection the structure of the stumps is of interest : Owing 

 to their positions, the left one is cut longitudinally and the right 

 one transversely in the sagittal series. The structure is the same 

 on the two sides (see Fig. 3) ; there is a single curved rod of pre- 

 cartilage extending out nearly to the tip of the limb ; this is sur- 

 rounded by a mass of dense mesenchyma, evidently premuscle 

 tissue, and externally are the elements of the skin. Now, in the 

 normal limb of corresponding age, the skeleton of the thigh, 



