8 LIBBIE H. HYMAN. 



anterior disintegration at about the middle of the somites. 

 From this time on a double gradient is present in the somites; 

 anterior and posterior somites are the most susceptible and from 

 them death proceeds towards the middle somites. 



11. Stage of Twelve Somites. In this, the stage of the classical 

 33-hour chick (Lillie, Fig. 63, p. 109), a new region of high 

 susceptibility makes its appearance, at the anterior end of the 

 hind brain. The disintegration of a twelve somite chick is 

 represented in Figs. 36 to 39. The high susceptibility of the 

 optic vesicles has now died away; evidently the process of 

 evagination is completed. Disintegration begins in the anterior 

 end of the primitive streak and the anterior end of the neural 

 axis and proceeds anteriorly from the former, caudally from the 

 latter. Very soon, however, the region in the hind brain men- 

 tioned above begins to disintegrate and from this region dis- 

 integration progresses in both directions along the neural tube as 

 shown in Fig. 37. This occurrence, at first puzzling, was readily 

 interpreted after later stages had been studied. It was then 

 realized that this region of high activity in the hind brain of 

 the twelve somite chick foreshadows physiologically the process 

 of turning of the head of the embryo which is not manifested 

 morphologically until a slightly later stage. 



12. Fourteen to Twenty Somites. The head of the embryo now 

 begins to turn on its left side and the stalks of the optic vesicles 

 narrow. Both of these processes are indicated in the dis- 

 integration. The disintegration of a fifteen somite chick is 

 shown in Figs. 40 to 44. Death begins in the anterior end of 

 the primitive streak and in the stalks of the optic vesicles. 

 From the former it progresses forwards along the neural folds and 

 segmental plate; from the latter laterally to the tips of the 

 vesicles. Disintegration next occurs at the anterior end of the 

 forebrain and in the hind brain; from these two regions it 

 progresses backward and forward respectively, meeting in the 

 region of the midbrain. The high susceptibility of the hind- 

 brain is evidently correlated with the turning of the head; it 

 should further be noted that the region of the brain involved in 

 this process has moved posteriorly as compared with the preceding 

 stage. In the remainder of the body disintegration proceeds 



