THE METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. I. TELEOST EMBRYOS

¹ HULL ZOÖLOGICAL LABORATORY, UNIVERSITY OF CHICAGO

1. The susceptibility of developing eggs of Fundulus, the cunner and the cod to toxic solutions at various stages was studied.

2. In early blastoderms the central region is more susceptible in Fundulus and the cunner, the peripheral region in the cod.

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 susceptibility 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 general susceptibility gradient in all three species is a "double" one. Anterior and posterior ends are the points of highest susceptibility and from them the susceptibility decreases in both directions 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 cerebellum.

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 rôle in development. The facts recorded do not support the theory of concrescence.

11. The oxygen consumption and carbon-dioxide production of developing eggs of Fundulus heteroclitus increase up to the time when the germ ring is at the equator of the egg. Subsequently they decrease relative to the amount of protoplasm but show an absolute increase owing to the heart beat and other activity. This period when the respiratory metabolism is greatest is also the period when the eggs are most readily modified by external agents.

12. The relation of the susceptibility data to teratological development is discussed at considerable length. It is shown that those parts of the embryo having the highest susceptibility are those which are most defective in teleost terata and that such differential susceptibility is therefore the explanation of teratological development. It is also shown that these same parts most susceptible under extreme conditions may recover if conditions permit and may develop while the less susceptible parts fail to recover. Recovery forms of terata are thus just opposite in appearance to inhibited types.

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