METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 5! 



unfortunately lacking except in the case of the salmon, in which 

 according to the experiments of Kopsch, most of the trunk and 

 tail of the embryo are produced through the activity of the por- 

 tions of the germ ring immediately adjacent to the early embry- 

 onic shield. It seems in view of the facts at hand legitimate to 

 draw the conclusion that in different teleost embryos the amount 

 of material contributed to the formation of the embryo by the 

 germ ring is variable. At one extreme are cases like Tautogola- 

 brus in which very little material is so contributed and at the 

 other extreme the case of Fundulus, where most of the embryo 

 is formed from a growing point in the germ ring. The disagree- 

 ment among investigators concerning the mode of formation of 

 the teleost embryo is evidently due to the circumstance that they 

 worked with different species. The mode of development in dif- 

 ferent species is not identical but rather exhibits various degrees 

 of modification from the primitive vertebrate type illustrated 

 by the ganoids and the amphibia to the extremely specialized 

 type in which the germ ring plays the dominant role. Owing to 

 this fact generalizations cannot be drawn from the study of a 

 single species and controversies which have arisen in the past 

 concerning the mode of origin of the teleost embryo are without 

 point. 



While it may be said that the evidence indicates that the germ 

 ring does add to the embryo in varying degrees in different 

 species, the facts recorded in this paper do not seem to me to 

 support the theory of concrescence. The embryo is not produced 

 by a concrescence of two areas of the germ ring even in cases like 

 the cod where the germ ring plays an important role in develop- 

 ment ; but there is present in the germ ring at the posterior end 

 of the embryonic axis a region of high activity which in some 

 species, like Fundulus, is of great importance in the formation 

 of the posterior part of the embryo. Some of the material of 

 the germ ring passes into the embryo but not in the manner re- 

 quired by the concrescence theory; and the anterior end of the 

 embryo is not formed in this way but by an independent region 

 of activity. 



