28 EARLY EMBRYOLOGY OF THE CHICK 



phalic end of the priinitive groove is deepened and often some- 

 what expanded to form a depression known as the primitive pit. 

 Directly anterior to the primitive pit the right and left primitive 

 folds merge with each other in the mid-line to form a small 

 rounded elevation called Hensen's node. Hensen's node is of 

 importance as a landmark rather than because it gives rise to 

 any particular structure. 



As early as the beginning of gastfulation the shape of the 

 blastoderm responds to local inequality in the rate of growth. 

 One of the early manifestations of differential growth is the 

 more rapid extension of the embryo cephalad than either 

 laterad or caudad. This results in a definite elongation in 

 the antero-posterior axis by the time the primitive streak is 

 established (Fig. 8). 



The Origin of the Primitive Streak by Concrescence of the 

 Blastopore. — The significance of the primitive streak has been 

 the subject of much controversy. The divergences of opinion 

 have been due chiefly to incomplete knowledge of the stages 

 of development passed through prior to the laying of the egg. 

 Our present knowledge of these early stages is, however, suffi- 

 cient to furnish the basis of an interpretation of the primitive 

 streak which is now widely accepted. This interpretation is 

 outlined below without reference to other, opposed views. 



The primitive streak is to be regarded as a scar-like thicken- 

 ing arising from the fusion of the edg^s_of jthe anterior lip of the 

 blastopore. To understand the origin of the longitudinally 

 placed primitive streak from the marginally located, crescentic 

 blastopore it is necessary to follow carefully the growth proc- 

 esses taking place during the closure of the blastopore. 



We have already seen how the ingrowth of entoderm from 

 the anterior lip of the blastopore, caused the blastopore to lag 

 behind the other parts of the margin of the blastoderm in the 

 process of radial extension over the yolk surface. During this 

 process the blastopore is compressed from either side toward 

 the mid-]ine by the rapidly extending margins of the blastoderm 

 adjacent to it and is eventually encompassed by them (see 

 Chap. IV and Fig. 7). Because of the insweeping, converging 

 tendency of the growth which first causes the blastopore to be 

 laterally compressed and finally causes its margins to grow 

 together the process has been termed concrescence. 



