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THE AMERICAN NATURALIST [Vol.XLIV 



... Die Entstehung eines Organs durcb Regeneration oder Knospung 

 keinen Anhaltspunkt giebt zur Beurteilung des Morpbologischen Wertes 

 seiner Entwicklung aus einem Keimblatt, d. h. die Verwertbarkeit der 

 Keimblatt-Herkunft eines Organs fur die Frage seiner Homologie mit 

 einem anderen Organ, dessen Keimblattursprung ebenfalls bekannt ist, 

 wird durch die Knospungs- und Regenerations-befunde in keiner Weise 

 beeintrachtigt. (L. Sehultze, p. 331.) 



The inextricable contradictions which sprang from the 

 tenets of historical and morphological significance of the 

 germ-layers called forth even a more vigorous reaction. 

 Braem subjected the theory to a thorough and relentless 

 critique revealing its inadequacy. "Der BegrifT Keim- 

 blatt," he proclaimed, "ist gar kein morphologischer 

 sondern ein physiologischer BegrifT. Keimblatter sind 

 Organbildner. " And further, he introduced the prag- 

 matic principle that 



... Ein Scbieht ist nieht deshalb Entoderm weil sic das innere Blatt 

 einer Gastrnla ist. sondern sie ist Entodenn weil sie den Darm bildet, 

 weil sie die physiologische Charaktere des Darmldatles entweder bereits 

 besitzt oder doch im Laufe der ferneren Entwicklung annimmt. . . . 

 Massgebend ist nur die organbildende Potenz, die Funktion der zell- 

 sehicht. (Braem, p. 431.) 



While embrvologists were thus questioning the impor- 

 tance of the germ-layer as a criterion and in fSct were 

 at a loss to know what was meant by a germ-layer, stu- 

 dents of the histogenetic processes in regeneration were 

 accumulating evidence to the effect that the new tissues 

 generally arise from corresponding old tissues, and, 

 consequently, from the same germ-layers from which 

 those tissues had been differentiated in development. 

 Although the evidence is based largely upon the investi- 

 gation of the regenerative process in worms, the facts 

 relating to this animal group as they are stated by vari- 

 ous authors are too much at odds with each other to be 

 of any real worth, at least so far as the problem of the 

 relation between regeneration and ontogeny is concerned. 

 This may seem a very strong statement, but it is justi- 

 fied by the frequent contradictions which one encounters 

 in reviewing the literature. To be specific, I might men- 



