Gastrulation in the Pigeon's Egg. 109 



blastoderm could be sufficient to overcome the resistance offered by 

 the hair held above by the vitelline membrane and below by the 

 yolk, even, indeed, if such material did not merely flow around the 

 obstructing hair. In the second place, both Kopsch and Assheton 

 were operating at a time when concrescence either had ceased alto- 

 gether, or its influence was waning. Thus in Kopsch's ('02) experi- 

 ment VII the operation was made after twelve hours of incubation, 

 at a time when the bulk of the axial material had been marshalled 

 from a lateral into a median position for a period of at least twelve 

 to fifteen hours. It is obvious, from the above morphological data, 

 that any experiments from which we could hope to gain any insight 

 into the part played by concrescence, must be made during gastru- 

 lation, for concrescence and gastrulation are but different phases 

 of the same process. 



If the avian embryo is the product of concrescence and the right 

 and left parts of the dorsal lip represent the homotypical halves 

 of the future embryo, then injuries made on the posterior margin 

 at different distances from the median line during early gastrula- 

 tion, ought later to appear at different levels in the embryo, that is, 

 an injury made at 10° from the median line ought to appear nearer 

 the head region than one made at 45°. Furthermore, such injuries 

 ought to affect only that half of the embryo which corresponds to 

 the side of the dorsal lip injured. The progress of concrescence can 

 be tested by operating on successively older stages. Thus, the fol- 

 lowing sets of operations will be described : Set A, on early gastrular 

 stages; Set B, on late gastrular stages; and Set C, on unincubated 

 and early incubation stages. 



Set a — On Eaely Gastrulae Stages. 

 Experiment III. 

 An injury made in the middle of the dorsal lip slightly within 

 the margin (Fig. XVI, a) is later found in the cephalic region of 

 the embryo, greatly disturbing the material of what is later to become 

 the primary fore-brain (Fig. 59, op). While the section through 

 this injured region shows the affected cells to be situated slightly 

 to the right of the median line, yet the entire head-fold is disturbed 



