SECT. 3] AND ORGANISATION 595 



the periphery of the blastoderm was more susceptible than the 

 central part. In the later blastoderm stages of the cunner the region 

 of high susceptibility is shifted posteriorly, and a certain area along 

 the margin of the blastoderm succumbs very readily indeed to the 

 toxic agent. This is exactly where the embryo is about to arise. The 

 eggs of the minnow could not be examined at this stage. In the 

 cod, the germ ring was always much more susceptible than the central 

 part of the blastoderm, and at its circumference one region is more 

 susceptible than the remainder. This is where the embryonic shield 

 originates, so that the conditions in the cunner and the cod are now 

 very similar. As the embryonic shield grows forward, its anterior 

 margin is most susceptible, and disintegration extends posteriorly 

 from this. 



Slightly later stages, when the embryo is visible in the centre of 

 the embryonic shield and the germ ring has advanced more than 

 half-way over the yolk, were not observable with certainty in the 

 cunner and the minnow. But in the cod they were clear enough, and 

 here toxic action obviously began at the anterior end of the embryo, 

 spreading backwards towards the posterior margin of the shield. 

 Still later, at the time of closure of the germ ring, disintegration 

 gradients were observable with ease in all three species, and always 

 the susceptibility was highest anteriorly, diminishing and spreading 

 backwards. The eyes are not very susceptible, and do not degenerate 

 until the wave has passed half-way back along the neural tube. 

 After the germ ring has closed, a secondary region of high suscepti- 

 bility appears at the posterior end of the embryo. From this point 

 onwards, there is no change in the gradients; there is a powerful 

 spreading backwards from the cephalic zone of high susceptibility 

 and a slight spread forwards from the caudal zone, with no compli- 

 cating factors. The minnow differs from the cunner in possessing 

 the two zones from the very earUest stages, and at certain early 

 points in development, the posterior zone is the more important 

 of the two. But as development proceeds the posterior zone declines 

 in susceptibility and the anterior one increases, especially after the 

 arrival of the optic vesicles, which fall an exceedingly easy prey to 

 the toxic agent. In very late stages in the minnow, a region of high 

 susceptibility develops in the hind brain where the cerebellum is 

 forming. By this time the tip of the tail has become free from the yolk 

 and somewhat more susceptible again. The cod embryo behaves in 



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