670 



MANUAL OF PHYSIOLOGY. 



gradually become lost. Between the two folds is a furrow lined 

 by epiblast, which is called the medullary groove.. 



The medullary folds growing upward turn in toward one 

 another, and eventually coalesce at their line of meeting, con- 



FIG. 260. 



Transverse section of the embryo of a chick at the end of the first day. (KOlliker.) 

 sp. Mesoblast. Pv. Medullary groove. Rf. Medullary fold. dd. Hypoblast. in. Me- 

 dullary plate, h. Epiblast. ch. Chorda dorsalis. uwp. Protovertebral plate, uwh. Di- 

 vision of mesoblast. 



verting the medullary groove into a channel the medullary 

 canal; this union of the folds takes place from before backward. 

 The medullary canal thus formed lies in the axis of the 

 embryo on the uncleft mesoblast ; it is covered in superficially by 

 several layers of epiblastic cells, which also line its walls. The 



FIG. 261. 



Transverse section of an embryo of a chick at the latter end of the second day. (KOl- 

 liker.) rw. Medullary fold. rf. Medullary groove. A. Epiblast. ao. Aorta, dd. Hypo- 

 blast, p. Pleuroperitoneal cavity, sp. External plate of mesoblast dividing, uwp. 

 Protovertebral plate. 



canal is *the earliest representative of the nervous centres, and 

 eventually becomes the brain and spinal cord. The front part 

 of the canal, when completely closed in, becomes dilated into a 

 bulb, thus forming the earliest indication of the brain. The 



