METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 9 



along the neural tube and somites from the two ends of the 

 body towards the middle, the two waves of disintegration meeting 

 at the middle of the somites. 



From this time on through more than twenty somites the dis- 

 integration gradients are about the same. The turning of the 

 head as well as the advancing head fold of the amnion obscure 

 observations on the disintegration of the head. The region of 

 high susceptibility in the hindbrain becomes more marked and 

 involves a larger area. The advancing margin of the head fold 

 of the amnion and also the auditory pits are regions of high 

 susceptibility. 



/j. Two-Day Chick. This stage, illustrated in Figs. 45 to 47 

 is marked by the high susceptibility of the eye. The formation 

 of the lens and the optic invagination are at their height. The 

 eye and the posterior end of the embryo are the most susceptible 

 parts. After the eye has disintegrated, disintegration begins in 

 the tip of the telencephalon and proceeds posteriorly along the 

 brain. A region of high susceptibility is also still present at 

 the place where the body is turning and from this place dis- 

 integration progresses in both directions. The auditory vesicles 

 disintegrate before the hindbrain. There is the usual postero- 

 anterior progress of the disintegration from the primitive streak. 



14. Two- to Three-Day Chicks. In later stages the region of 

 high susceptibility at the bend of the embryo gradually dis- 

 appears; it can still be detected at the beginning of the third day 

 of incubation but subsequently vanishes. The susceptibility of 

 the posterior end of the embryo also gradually diminishes. In 

 the two-day chick the tail bud is still the most susceptible part 

 of the embryo, but in the three-day chick, the tail bud does not 

 begin to disintegrate until sometime after anterior structures 

 have undergone disintegration. Of anterior structures the eye is 

 by far the most susceptible in these later stages. Disintegration 

 begins in the lens vesicle and then extends to the optic cup. 

 Next the telencephalon disintegrates and this process proceeds 

 posteriorly along the brain. The gradient of the brain is rather 

 shallow, however. At all stages which were observed the double 

 gradient was present but the susceptibility of the posterior end 

 as compared with the anterior end diminishes in the later stages, 



