34 LIBBIE H. HYMAN. 



PLATE I. 



FIGS, i TO 3. Three stages in the disintegration of a chick embryo in the 

 primitive streak stage showing course of disintegration from anterior to posterior 

 end of the streak. 



FIGS. 4 TO 6. Chick embryo in the head process stage, Fig. 4, disintegration 

 of the head process, Fig. 5, and course of disintegration posteriorly along the streak, 

 Fig. 6. 



FIGS. 7 TO 10. Medullary plate stage. Disintegration begins in the primitive 

 knot, Fig. 7, and progresses posteriorly along the streak, Figs. 8 and 9. It then 

 attacks the anterior end of the medullary plate, Fig. 9, and proceeds backward 

 along the plate, Fig. 10. 



FIGS, ii TO 13. Early neural fold stage. Disintegration similar to the pre- 

 ceding. 



FIGS. 14 TO 17. Stage of the first somite. Disintegration begins in the 

 anterior end of the primitive streak, Fig. 15, and proceeds in both directions from 

 this region. It then attacks the anterior end of the neural folds, Fig. 16 and 

 progresses posteriorly meeting the other disintegration, Fig. 17. 



FIGS. 18 TO 20. Three somite embryo. Disintegration begins in the differenti- 

 ating anterior end of the primitive streak and in the middle of the neural folds. 

 From both places it progresses forward and backward. The high susceptibility 

 of the middle of the neural folds foreshadows the approaching fusion of these 

 folds. 



FIGS. 21 TO 25. Embryo of five somites, showing the double gradient, from 

 the primitive streak forward and from the anterior end of the neural axis backward. 



FIGS. 26 AND 27. Embryo of five somites in which the place of fusion of the 

 neural folds is evidenced by heightened susceptibility. 



