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 primitive 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- 

 wards. 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- 

 wards to overlap it at the 

 sides, arise two folds of the 



View of the area pellucida of a chick 

 of eighteen hours seen from above. 

 (Foster and Balfour.) A. Medullary 

 folds. me. Medullary groove, pr, 

 Primitive streak and groove. 



epiblast, called the laminae 

 dorsales, or the medullary 

 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 



