CHAPTER V. 



THE CHANGES WHICH TAKE PLACE DURING THE 

 SECOND HALF OF THE SECOND DAY. 



ONE important feature of this stage is the rapid 

 increase in the process of the folding-off of the embryo 

 from the plane of the germ, and its consequent con- 

 version into a distinct tubular cavity. At the begin- 

 ning of the second day, the head alone projected from 

 the rest of the germ, the remainder of the embryo 

 being simply a part of a flat blastoderm, nearly com- 

 pletely level from the front mesoblastic somite to the hind 

 edge of the pellucid area. At this epoch, however, a 

 tail-fold makes its appearance, elevating the tail above 

 the level of the blastoderm in the same way that the 

 head was elevated. Lateral folds also, one on either 

 side, soon begin to be very obvious. By the progress 

 of these, together with the rapid backward extension 

 of the head-fold and the slower forward extension of 

 the tail- fold, the body of the embryo becomes more and 

 more distinctly raised up and marked off from the rest 

 of the blastoderm. 



The medullary canal closes up rapidly. The wide 

 sinus rhomboidalis becomes a narrow fusiform space, 



