METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 65 



3. In late blastoderms, the median posterior region of the germ 

 ring where the embryonic shield is to arise is the most susceptible 

 region. 



4. After the formation of the shield, its anterior portion is the 

 most susceptible. 



5. After the origin of the embryonic axis the anterior end of 

 the axis is the most susceptible and from this point the suscep- 

 tibility decreases posteriorly. 



6. Sooner or later a secondary region of high susceptibility 

 arises at the posterior end of the embryo. This secondary region 

 arises very early in Fundulus, later in the cod, and very late in 

 the cunner. 



7. After the origin of the secondary posterior region, the gen- 

 eral susceptibility gradient in all three species is a " double " one. 

 Anterior and posterior ends are the points of highest suscep- 

 tibility and from them the susceptibility decreases in both direc- 

 tions towards the middle. Both ectodermal and mesodermal 

 structures (somites) are involved in the double gradient but the 

 ectodermal structures (neural tube) are in general much more 

 susceptible. 



8. The heart is highly susceptible (Fundulus). The venous 

 end of the heart is the most susceptible part of it and from it the 

 susceptibility decreases towards the arterial end. 



9. Besides the general gradients, specific organs may exhibit 

 high susceptibility. Conspicuous examples of this are the eyes 

 (especially in Fundulus), the auditory vesicles, and the cere- 

 bellum. 



10. The relations of these gradients to normal development 

 are considered. It is pointed out that the embryo arises for the 

 most part from material that does not come from the germ ring 

 but that later the germ ring contributes to the embryo in degrees 

 varying in different species. It is further pointed out that the 

 germ ring type of development is probably a specialization from 

 a method in which the center of the blastoderm played the chief 

 role in development. The facts recorded do not support the 

 theory of concrescence. 



11. The oxygen consumption and carbon-dioxide production 



