774 HANDBOOK OF PHYSIOLOGY. 



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 collection of mesoblastic cells from the hypoblast in the middle 

 line, and remains connected with the latter after the lateral portions of 

 the mesoblast have become quite detached from it. The notochord and 

 the hypoblast 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 collection 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. Although behind the groove is a shallow one, if it 

 be traced forward it becomes deeper and narrower, and at the headfold 

 the folds curve round and meet in the middle line. Anterior to the 

 headfold is a second fold parallel to it, which is the commencing amnion. 



^ ^^fe 1 ^^^ &: * 





Fig. 466. Vertical section of blastoderm of chick (1st day of incubation). S, epiblast con- 

 sisting of short columnar cells ; Z), hypoblast, consisting of a single layer of flattened cells ; M , 

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

 hypoblast to form the mesoblast; A, white yolk granules. Many of the large "formative cells" 

 are seen containing these granules. (Strieker.) 



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

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

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

 464) 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 prim- 

 itive 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 dorsalis takes place first 

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

 region of the head, while the closing in of the groove progresses much 

 more slowly toward 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 ante- 



