684: 



HANDBOOK OF PHYSIOLOGY. 



streak an opaque line, which proceeds almost to the anterior edge of the 

 area pellucida, stopping short at a transverse crescent-shaped line, the 

 future headfold. This line is the commencing notochord. It is a col- 

 lection of mesoblastic cells from the hypoblast in the middle line, and 

 remains connected with the latter after the lateral portions of the meso- 

 blast have become quite detached from it. The notochord and the hypo- 

 blast from which it arises are continued posteriorly into the primitive 

 streak. Thus the mesoblast of the area on either side of the middle line 

 in which the embryo is formed arises from the hypoblast, as does also 

 the notochord. In the formation of the medullary plate which now 

 appears, the epiblast is concerned. In the middle line above the collec- 

 tion of cells that will become the notochord that layer becomes thickened. 

 The sides of the central thickened portion are elevated somewhat to form 

 the medullary folds, inclosing between them the medullary groove. 

 From this medullary plate is formed the central nervous system. Al- 

 though behind the groove is a shallow one, if it be traced forwards it 

 becomes deeper and narrower, and at the headfold the folds curve round 





FIG. 448. Vertical section of blastoderm of chick (1st day of incubation). 8, epiblast, consist- 

 ing of short columnar cells; Z>, hypoblast, consisting of a single layer of flattened cells; M, "for- 

 mative cells." They are seen on the right of the figure, passing in between the epiblast and hypo- 

 blast to form the mesoblast; A, white yelk granules. Many of the large ' formative cells " are seen 

 containing these granules (Strieker). 



and meet in the middle line. Anterior to the headfold is a second fold 

 parallel to it, which is the commencing amnion. 



* The medullary canal is bounded by its two folds or longitudinal eleva- 

 tions, laminae dorsales, which are folds consisting entirely of cells of 

 the epiblast: these grow up and arch over the medullary groove (Fig. 

 446) till after some time they coalesce in the middle line, converting it 

 from an open furrow into a closed tube the neural canal or the primi- 

 tive cerebro-spinal axis. Over this closed tube, the walls of which con- 

 sist of more or less cylindrical cells, the superficial layer of the epiblast 

 is now continued as a distinct membrane. 



The union of the medullary folds or laminae dorsales takes place first 

 about the neck of the future embryo; they soon after unite over the re- 

 gion of the head, while the closing in of the groove progresses much more 

 slowly towards the hinder extremity of the embryo. The medullary 

 groove is by no means of uniform diameter throughout, but even before 

 the dorsal laminae have united over it, is seen to be dilated at the anterior 



