120 THE STRUCTURE OF THE FOWL 



later become the mid-brain or the anterior part 

 of the hind-brain. From this point the con- 

 version of the groove into the neural tube 

 spreads forwards and backwards, but is not 

 completed posteriorly until after the entire 

 disappearance of the primitive streak. The 

 final point of closure at the anterior end of 

 the embryo is known as the neuropore and is 

 regarded as of great morphological importance. 



During the early stages of development 

 there is no distinction between the limits of 

 the embryo and the rest of the blastoderm. 

 Before the end of the first day, however, a 

 crescentic infolding, or head -fold, makes its 

 appearance at the head end of the embryo. 

 Tliis, and a similar but shallower tail-fold with 

 a pair of lateral folds finally produce a con- 

 tinuous furrow all round the embryo, which is 

 thus marked off from the rest of the blastoderm 

 (Figs. 63, 64, and 65). From that part of the 

 blastoderm outside the furrow extra-embryonic 

 membranes are produced. By this time the 

 notochord, a very important structure char- 

 acteristic of all vertebrate embryos and the 

 forerunner of the vertebral column, has made 

 its appearance beneath the neural groove. 



Orientation of the Embryo.' — The developing 



