130 Vertebrate Embryology 



end of the first day, become very pronounced, 

 and in the region of the future brain (the 

 medullary folds, as was described in connec- 

 tion with the frog, are the beginning of the 

 nervous system) they arch over until they 

 meet in the middle line to enclose the medul- 

 lary canal, though they do not actually fuse 

 together until a somewhat later period. The 

 appearance of the medullary folds at the end 

 of the first day of incubation, as seen from 

 the surface, is shown in Fig. 44 (A}. It will be 

 noticed in this figure that, while the folds are 

 nearly in contact for a considerable part of 

 their length, they are still widely separated 

 at the extreme anterior end, where a tri- 

 angular space is left : and at the posterior 

 end the two folds diverge widely, and grad- 

 ually diminish in height as they pass back 

 on each side of the primitive streak. At the 

 extreme anterior end the two folds are con- 

 tinuous with each other, across the base of 

 the triangle mentioned above. 



During the second half of the first day the 

 head-fold makes its appearance, and, by the 

 end of the day, it has progressed so far (Fig. 

 44, A), that the head region of the embryo 

 is distinctly marked off. The formation of 



