28o Vertebrate Embryology 



a considerable amount of fluid. Slow, rhythmic 

 pulsations are seen passing over the amnion, 

 caused, probably, by the contraction of the 

 muscle-fibres that are developed in the meso- 

 blastic portion of the amniotic folds. This 

 pulsation of the amnion gives to the embryo a 

 sort of rocking motion in the amniotic fluid. 

 At a later period, such movements, it is said, 

 may be seen in the allantois. 



The allantois has increased rapidly in size, 

 and contains a quantity of fluid. Both the 

 vitelline veins and arteries now pass from the 

 body as single vessels ; and the yolk, though 

 apparently undiminished in quantity, is much 

 more fluid than during the earlier days. The 

 flexure of the body is less marked than on 

 previous days ; and the neck is becoming 

 more apparent. The disproportion between 

 the size of the head and that of the rest of the 

 body is being reduced by the more rapid 

 growth of the body. The width of the so- 

 matic stalk is being reduced, with the result 

 that the heart is being enclosed, and no longer 

 seems to hang loosely out of the body : in the 

 wall of the body thus formed, the cartilaginous 

 ribs and sternum are being developed. The 

 growth of the cerebral hemispheres has been 



