36 LIBBIE H. HYMAN. 



PLATE II. 



FIGS. 28 TO 30. Embryo of seven somites, showing the usual double gradient 

 of disintegration. 



FIGS. 31 TO 35. Embryo of ten somites, showing the double gradient of 

 disintegration and increasing susceptibility of the optic evaginations. 



FIGS. 36 TO 39. Embryo of 12 to 13 somites showing general double gradient 

 and area of increased susceptibility in the anterior end of the hindbrain fore- 

 shadowing the turning of the head. 



FIGS. 40 TO 44. Fifteen somite embryo, with general double gradient; in- 

 creased susceptibility of the optic stalks; increased susceptibility in the hind 

 brain, more posteriorly located than in preceding figures, correlated with the 

 turning of the head. 



