672 



MANUAL OF PHYSIOLOGY. 



fold, whilst posteriorly they diverge, and, passing on either side 

 of the primitive groove, gradually become lost. Between the two 



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. 

 d.d. Hypoblast. m. Medullary plate, h. Epiblast. ch. Chorda dorsalis. 

 uwp. Protovertebral plate, uwh. Division of mesoblast. 



folds is a furrow lined by epiblast, which is called the medullary 

 groove. 



The medullary folds growing upwards turn in towards one 

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



Transverse section of an embryo of a chick at the latter end of the second 

 day. (Kolliker.) rw. Medullary fold. rf. Medullary groove. A. Epi- 

 blast. ao. Aorta, dd. Hypoblast. p. Pleuro-peritoneal cavity, sp. Ex- 

 ternal plate of mesoblast dividing, uwp. Protovertebral plate. 



verting the medullary groove into a channel the medullary 

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

 The medullary canal thus formed lies in the axis of the em- 

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

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



