CHAPTER XXXVIII. 



FIG. 259. 



DEVELOPMENT OF THE SPECIAL SYSTEMS. 

 DEVELOPMENT OF THE VERTEBRAL Axis. 



The earliest evidence of the differentiation of the blastoderm 

 consists in the appearance of the primi live streak which forms the 

 first sign of the embryo. This 

 is a line which appears near 

 what is to be the tail end of 

 the embryo, and runs for- 

 ward. This primitive line 

 or streak is due to the thick- 

 ening of the mesoblast, and 

 it becomes converted into a 

 groove by a depression ap- 

 pearing in its centre, forming 

 the primitive groove. This 

 extends in a forward direc- 

 tion, but never reaches the 

 head fold of the embryo, 

 which, in the chick, appears 

 a few hours after the forma- 

 tion of the primitive groove. 



In front of the primitive 

 groove, and stretching back- 

 ward to overlap it at the 



sides, arise tWO folds of the View of the area pellucida of a chick of 



PnfhW rallpf! thp Inmin^ eighteen hours seen from above. (Foster and 



lSt ' ( ' Balfour.)-^. Medullary folds, me. Medul- 



dorsaleS, Or the medullary lary groove, pr. Primitive streak and groove. 



folds. 



These are the elevations of the epiblast, beneath which the 

 mesoblast is thickened. They arise in front, where they are 

 joined immediately behind the head fold, while posteriorly they 

 diverge, and, passing on either side of the primitive groove ? 



669 



