6 LIBBIE H. HYMAN. 



along the folds, meeting the disintegration which is advancing 

 forward from the somitic region. The high susceptibility of the 

 lateral boundaries of the embryo is also noticeable. 



8. Stage of Three or Four Somites. The disintegration of a 

 three somite embryo is shown in Figs. 18 to 20. The death 

 changes begin as before at the anterior differentiating end of the 

 primitive streak and proceed in both directions from this region. 

 A region of high susceptibility is now, however, noticeable in 

 the center of the neural folds, as shown in Fig. 18. From this 

 place disintegration proceeds in both directions along the neural 

 folds. The explanation of this circumstance is simple the 

 neural folds are in process of fusion. As is well known, this 

 fusion does not begin at the anterior end of the neural axis but 

 posterior to this point in a region corresponding according to 

 Lillie, p. 99, "approximately with the region of the future mid- 

 brain or anterior part of the hind brain." There is thus an 

 increased activity preceding the closure of the neural folds for the 

 neural folds meet "by the time four or five somites are formed" 

 (Lillie, p. 98) while the increased susceptibility of this region 

 is already evident at a three somite stage. The somites at 

 this time disintegrate from the segmental plate forward. The 

 lateral boundaries of the embryo are also highly susceptible; 

 processes are evidently in progress there in connection with the 

 separation of the embryo from the blastoderm. The disintegra- 

 tion of four somite embryos is the same as that of three somite 

 stages. 



9. Five to Eight Somites. The region of high susceptibility at 

 the place of closure of the neural folds persists for a varying 

 length of time. Embryos with as many as seven or eight somites 

 may still exhibit traces of it. In such later embryos this region 

 of initial disintegration of the neural tube has moved backward 

 into the hindbrain. Sooner or later, however, this region of high 

 susceptibility in the hindbrain region disappears and the dis- 

 integration gradient of the brain region is again of the simple 

 antero-posterior type. Five somite embryos may have already 

 reached this condition. Such an embryo is illustrated in Figs. 

 21 to 25. Disintegration begins as before in the clavate differ- 

 entiating region of the primitive streak and proceeds in both 



