METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 41 



gradient is the same except that a secondary region of high sus- 

 ceptibility is faintly evident at the posterior margin of the shield. 

 This stage is depicted in Figs. 22 to 26. 



5. Disintegration of Later Stages of the Embryo. After the 

 embryo had become established its disintegration gradient was 

 observed with ease in all three species and in a great many indi- 

 viduals. In Tautogolabrus disintegration begins at the anterior 

 end of the embryo and proceeds posteriorly along the neural tube 

 to its posterior end. An early embryo is depicted in Figs. 27 

 and 28 and a later one in Figs. 29 to 32. The disintegration 

 gradient remains the same up to the time when the germ ring 

 closes. The eyes are not very highly susceptible but disintegrate 

 at about the time when the disintegration in the neural tube has 

 extended half way back. The neural tube usually separates from 

 the rest of the embryo and becomes arched. The time of death 

 of the somites could not be determined with certainty as they do 

 not seem to undergo disintegration but remain distinct long after 

 the disintegration of the neural tube is completed. After the 

 germ ring has closed a secondary region of high susceptibility 

 appears at the posterior end of the embryo as shown in Figs. 33 

 and 34. From this time on there is no further change in the 

 disintegration gradients ; there are always two regions of high 

 susceptibility, one at each end of the embryo ; disintegration pro- 

 ceeds posteriorly along the neural tube and anteriorly to a slight 

 extent from the end of the tail. In no case was the fate of 

 the somites determined. 



In Fundulus embryos the disintegration gradients are in general 

 the same as in Tautogolabrus with certain exceptions. In Fundu- 

 lus the two regions of high susceptibility are present from th 

 earliest observable stages of the embryonic axis. The posterior 

 end of the embryo is the more susceptible and disintegration begins 

 there, and progresses anteriorly. Disintegration then begins at 

 the anterior end of the embryo and progresses posteriorly. In 

 the very earliest stages as in Fig. 19 this anterior disintegration 

 begins at the tip of the neural axis. Very soon, however, the 

 optic bulbs make their appearance. As soon as this has occurred, 

 the optic bulbs are decidedly the most highly susceptible parts of 



