48 LIBBIE H. HYMAN. 



PLATE I. 



FIGS, i TO 3. Ventral view of anterior end of an embryo of five to six somites, 

 showing the disintegration of the crest of the head fold. 



FIGS. 4 TO 8. Five stages in the disintegration of a heart of a seven somite 

 chick, showing the sin-arterial gradient with a slight reversal at the arterial end. 



FIGS. 9 TO ii. Course of disintegration of an eight somite heart, similar to 

 the preceding, but with a more extensive reversal at the arterial end. 



FIGS. 12 TO 16. Disintegration of a nine somite heart, showing general sin- 

 arterial gradient and greater susceptibility of the right side as compared with 

 the left. 



FIGS. 17 TO 21. Disintegration of a ten somite heart, similar to the preceding, 

 the greater susceptibility of the right side very marked. 



