METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 43 



picted in Fig. 37. The disintegration gradient is much the same 

 as described in the preceding paragraph except that a region of 

 high susceptibility has developed in the hindbrain where the 

 cerebellum is forming. The end of the tail has by this time 

 become free from the yolk and is elongating; its susceptibility is 

 increased relative to the preceding stage. The tip of the fore- 

 brain and the eyes are about equally susceptible and from them 

 disintegration proceeds posteriorly along the axis in both neural 

 tube and mesoderm. The most anterior and posterior somites 

 are more susceptible than the others and from them disintegra- 

 tion proceeds in both directions to the middle of the embryo. 

 Investigation of later stages of Fundulus embryos was not feas- 

 ible as the embryos do not disintegrate as readily as previously. 

 It may be stated, however, that the auditory vesicles and the buds 

 of the pectoral fins were observed to be regions of high sus- 

 ceptibility. 



The later stages of the cod embryo resemble those of Fundulus 

 except with reference to the eyes. There are always two regions 

 of high susceptibility, the anterior and posterior ends of the 

 embryonic axis. The anterior end commonly precedes in stages 

 before the closure of the germ ring. Disintegration begins at the 

 tip of the forebrain and passes backwards along the brain and 

 eyes; next the germ ring at the posterior end of the embryonic 

 axis disintegrates and this disintegration extends forwards ; the 

 two waves of disintegration meet anterior to the middle of the 

 embryo. The eyes are not highly susceptible as in Fundulus but 

 about as susceptible as the forebrain. This stage is illustrated in 

 Fig. 38. As the germ ring closes the susceptibility of the pos- 

 terior end increases and the eyes become temporarily more sus- 

 ceptible than the forebrain, as in the stage depicted in Fig. 39, 

 where the germ ring is on the point of closure. A later stage is 

 illustrated in Fig. 40. The susceptibility of the eyes has de- 

 creased but otherwise the disintegration gradient is the same as 

 in the preceding stage. The disintegration of the somites as 

 shown in Figs. 39 and 40 is the same as in Fundulus, the disin- 

 tegration proceeding from each end to the middle. In both 

 - 39 an d 40 a region of high susceptibility exists at the level 



