Gastnilation in the Pigeon's Egg. 93 



is sufficient to state here that in all probability it is not brought 

 about alone by the multiplication of cells in situ. 



A section lateral to the median line shows essentially the same 

 conditions as Fig. 34, except that the entoderm does not extend so 

 far anteriorly (Fig. 33, e). 



The question must naturally arise in the reader's mind as to 

 whether or not the upper layer is still rolling under at the posterior 

 margin to give rise to the lower layer. The appearance of sections 

 would seem to indicate that it is (e. g.. Fig. 33). The question can 

 be tested, however, by experimentation, for if a rolling under is 

 occurring, cells disturbed by an injury made on the extreme posterior 

 margin of the dorsal lip, ought to be found later in the entoderm. 



op 



Fig. X. Scheme for operating in Experiment II. 



Experiment II. — The scheme for such an operation is shown in 

 Fig, X, and the result in Fig. 50. The injured cells are found im- 

 mediately associated with the entoderm. This is especially clear 

 in a transverse section through the affected region (Fig. 51). There 

 is no evidence of an injury either in the ectoderm or mesoderm, 

 and hence we must conclude that the affected cells have been brought 

 to their present position by an actual rolling under of the posterior 

 margin. Although this operation has been repeated several times 

 with the above result, yet the position of the injury in the entoderm 

 may vary in an antero-posterior direction ; but this variation is 

 easily accounted for by the fact that one can tell in the living egg 

 only approximately the extent to which invagination has progressed. 



If an injury be made in the same manner as above on slightly 



