METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. " 



directions, anteriorly along the posterior ends of the neural 

 folds and the segmental plate, posteriorly to the caudal end of 

 the embryo. Disintegration next attacks the anterior end of the 

 neural tube and progresses posteriorly meeting the other dis- 

 integration at the level of the first pair of somites. In another 

 embryo of five somites, the region of high susceptibility in the 

 hindbrain, correlated with the closure of the neural folds, was 

 still present. The disintegration of the neural tube of this 

 embryo is shown in Figs. 26 and 27; otherwise it was identical 

 with the embryo represented in Figs. 21 to 25. 



The disintegration of an embryo of seven somites, corre- 

 sponding to Lillie's Fig. 59, p. 104, is illustrated in Figs. 28 to 30. 

 Death changes begin as always in the clavate differentiating 

 region of the primitive streak and proceed anteriorly from here 

 along the segmental plate and posterior ends of the neural folds. 

 They then begin in the fore-brain, in the optic evaginations, and 

 progress posteriorly along the neural axis. Other embryos of 

 seven and eight somites may still exhibit barely perceptible 

 traces of heightened susceptibility in the region where the neural 

 folds are closing. 



In all embryos up to seven or eight somites the gradient in 

 the somites is, as far as could be determined with certainty, from 

 the segmental plate forward. It is somewhat difficult to observe 

 the somites in these early stages owing to the fact that the 

 neural folds as they die spread open and conceal the somites from 

 view. In a number of cases, however, the death of the somites 

 was observed with certainty and is, as stated, from the segmental 

 plate anteriorly. 



10. Nine to Eleven Somites. The chief point of interest about 

 embryos of these ages is the rapidly increasing susceptibility of 

 the optic evaginations. The death of an embryo of eleven 

 somites is illustrated in Figs. 31 to 35. Disintegration begins 

 in the anterior end of the primitive streak, as usual, and simul- 

 taneously in the tips of the optic vesicles. From the first 

 named region it progresses anteriorly along the neural folds and 

 segmental plate. The optic vesicles disintegrate from their 

 lateral extremities medially; disintegration then passes pos- 

 teriorly along the neural tube meeting the advancing postero- 



