CH. LVIII.] THE PROTOVERTEBR.E. 825 



While these changes have been going on, the ventral walls of 

 the embryo have been formed by the downgrowth of the cephalic 

 fold in the head region, the caudal fold at the tail end of the 

 animal, and the two lateral or umbilical folds which grow last and 

 enclose the thoracic and abdominal organs. 



The embryo also undergoes certain changes in form and 

 attitude; in the first place torsion takes place; this is more 

 marked in birds and reptiles than in mammals ; by this term one 

 means that the embryo no longer lies ventral surface downwards 

 facing the yolk-sac, but turns slightly over so that the left side is 

 lowermost ; in birds the embryo may turn through a quarter of a 



Fig. 642. A human embryo of the fourth week, 3j lines in length. i, the chorion ; 3, part 

 of the amnion ; 4, umbilical vesicle with its long pedicle pawing into the abdomen ; 7, 

 the heart ; 8, the liver ; 9, the visceral arch destined to form the lower jaw. beneath 

 which are two other visceral arches separated by the branchial clefts ; ip, rudiment of 

 the upper extremity; n. Unit of the lower extremity; 12, the umbilical cord; 15, 

 the eye; 1 6, the ear; 17, cerebral hemispheres; 1 8, optic lobes, corpora quadrigemina. 

 (Mttller.) 



circle. Then the vertebral column becomes curved, but the chief 

 bend is known as the principal cephalic flexure. This occurs at 

 the anterior end of the notochord ; it is a strong angular flexion 

 in the region of the mid-brain, which is subsequently the position 

 of the sdla turcica. 



In connection with this must be mentioned the development of 

 the pituitary I tody which occupies the sella turcica in the adult. 

 It is formed by the meeting of two out-growths, one from the 

 foetal brain, which grows downward, and the other from the 

 epiblast of the buccal cavity, which grows up towards it. The 

 surrounding mesoblast also takes part in its formation. The 

 connection of the first process with the brain becomes narrowed, 

 and persists as the infundibulum, while that of the other process 

 with the buccal cavity disappears completely. 



