1?A NATURAL SCIENCE [February 



praeoral trochal ring in the Annelid Trochosphere-larva, and his 

 comparison of it with the circular nerve of a Medusa, may perhaps 

 be attributed to an effort of genius. Kleinenberg's recognition of 

 the probability of the >J< shaped ' Anlage ' of the mesoderm of the 

 Ctenophora indicating the former occurrence of tentacles in the 

 transverse plane, in addition to those which actually occur in the 

 sagittal plane, is only one other instance of his morphological 

 acumen. 



The memoir on Lopadorhynchiis appeared at a time when that 

 aspect of thought which may be denominated Haeckelism was 

 more or less in the ascendant — in other words, when morphology 

 was, for the time being, bound by the fetters of the gastraea theory, 

 and the coelora theory, and the germ-layer theory. It should not be 

 necessary to add that it is farthest from my thoughts to depreciate 

 the value of these most useful and indispensable theories. But no 

 one can deny that it is very important to guard against the possi- 

 bility of theory becoming dogma. Therefore, when a man of 

 Kleinenberg's timbre boldly proclaimed " Es giebt kein Mesoderm," 

 and, furthermore, undertook to substantiate his position by what 

 will ever remain a masterpiece of embryological research, the effect 

 could only be salutary as giving morphologists pause. 



With regard to the specific assertion quoted above, Kleinenberg's 

 conclusions have no doubt had to submit to important modifications 

 {cf. Ed. Meyer, "Die Aljstammung der Anneliden," Biol. Gentralh., 

 vol. X., 1890, p. 296; and E. B. Wilson, " The Origin of the Meso- 

 blast Bands in Annelids," Journ. Morph., vol. iv., 1891, p. 205). 

 That the value of a man's work is not, however, necessarily to be 

 measured so much by its intrinsic, theoretical accuracy as by the 

 effect which it has upon the suVyect as a whole and upon thought, 

 is amply demonstrated by the vicissitudes to which even the Theory 

 of Natural Selection is constantly exposed in the minds of natural- 

 ists. 



That Kleinenberg himself smarted under the restrictions imposed 

 by the too dogmatic laws and principles of certain authors seems to 

 be revealed in his perhaps rather impatient mot, "Loch ist Loch 

 in der ganzen Welt." 



If his wholesale repudiation of the mesoderm cannot be upheld, 

 there can be no doubt as to the lasting value of his principle of 

 substitution as applied to the interpretation of embryological data. 



Long before the appearance of Wilson's admirable work on 

 AmpJdoxus (E. B. Wilson, " On Multiple and Partial Development 

 in Am2)hioxus," Anat. Anz., vol. vii., 1892 ; also, " Amphioxus and 

 the Mosaic Theory of Development," Journ. Morph., vol. viii., 1893), 

 Kleinenberg was conscious of the fact that the so-called mesodermic 

 pole-cells of Amphioxus had no definite existence, and his scepticism 

 as to the independent rights of the mesoderm may well have been 

 enhanced by the conviction that structures of such fundamental 

 significance, from the point of view of the germ-layer theory, in such 

 an important type as Amphioxus, were mythical. 



The " New Morphology " has not yet come to birth, but when it 

 does see the light, its ancestry will probably be traced to such a man 

 as Kleinenberg. Arthur Willey. 



