Gastrulation in the Pigeon's Egg. Ill 



find that the injury is situated seven sections anterior to the posterior 

 end of the resulting embryo (Fig. 71 op), that is, the injured cells 

 'have been moved from a lateral into a median position. In the 

 sketch of the transverse section through the affected region (Fig. 

 XVII) the mass of cells is seen to be located slightly to the right side. 

 While the needle destroyed a considerable portion of the primitive 

 streak material, yet the blastoderm has apparently recovered from 

 the injury, with the mass of affected cells separated from the blasto- 

 derm proper. 



The apparent recovery of the blastoderm from the operation is 

 to be explained by the fact that the injury, being made so far to 

 the side, affected a region less highly differentiated than in the case 

 of the operation at ten degrees. That is what one would expect. 



Fig. XVII. Transverse section through the injured region of the embryo 

 shown in Fig. 71. See text for description, x ^^■ 



for just as at any given period of the early development, the anterior 

 portion of the embryo (or primordium) is in a higher state of differ- 

 entiation than the posterior. 



The results of the above experiments (III-V) show very clearly 

 that the axial portion of the embryo arises from material previously 

 situated in the right and left halves of the dorsal lip, material brought 

 together by the process of concrescence. These experiments have 

 been repeated several times, both in the manner described above, 

 and with certain variations. Thus an operation made on the lip 

 at twenty degi'ees either to the right or to the left of the axial line, 

 19 later found at the level of the anterior somites. In this paper 

 we are not concerned, however, in analyzing the exact morphological 

 value of the different parts of the blastoporic lip, but rather in show- 

 ing that the avian embryo is the product of concrescence. 



If the theory of concrescence is correct, it is obvious from these 



