METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 39 



this region disintegration extends forward along the margins of 

 the blastoderm. In later stages when the germ ring is approach- 

 ing the equator of the egg, the region of high susceptibility is 

 shifted still more posteriorly. At this time the observations are 

 somewhat obscured by the occurrence already described for 

 Fundulus. The blastoderm, which covers nearly half of the 

 egg, as shown in Fig. 9, breaks loose at its periphery and shrinks 

 and arches up from the yolk, as in Fig. 10. Nevertheless the 

 course of disintegration was followed in a number of individuals. 

 A certain area along the margin of the blastoderm is more sus- 

 ceptible than any other region, as illustrated in Fig. 10; from 

 this area disintegration spreads forwards and laterally as in Fig. 

 ii. There cannot be any doubt that the region of highest suscept- 

 ibility is the place where the embryonic axis is to arise. In 

 Tautogolabrus by the time that the blastoderm has spread nearly 

 half way around the yolk, the eggs float in a tilted position so that 

 half of the blastoderm is on the upper and half on the lower 

 side. The embryo arises in the center of the lower half. It is 

 therefore a simple matter to determine in eggs of the stage de- 

 picted in Fig. 9 where the embryo is to arise. The place where 

 this occurs is the region of the high susceptibility shown in 

 Fig. ii. 



Observations on the late blastoderm stages of Fundulus were 

 impossible owing to the behavior of the blastoderm. It bursts at 

 some point opposite the place where the embryonic shield is form- 

 ing; it then shrinks rapidly forming a mass about the embryonic 

 shield. In spite of repeated attempts it was impossible to come to 

 any conclusions regarding the susceptibility relations at these 

 stages owing to this shrinkage. 



In the eggs of the cod, the germ ring is differentiated at a very 

 early stage. The germ ring is always more susceptible than the 

 central part of the blastoderm and one region is more susceptible 

 than the remainder of its circumference. The disintegration is 

 therefore the same as before the germ ring has become visible 

 and as depicted in Fig. 6 and 7. There can be no reasonable 

 doubt that the point of high susceptibility in the germ ring is the 

 place where the embryonic shield originates. The shield soon 



