350 Prof, P. J. Van Bcnedcn on the Intestinal Worms. 



In conversation^ however, it has often seemed to us that the 

 term is better known than the essence of the facts and the ge- 

 neral views founded on them; otherwise " Generationswechsel" 

 would not, as I remember once to have read and sometimes to 

 have heard, be confounded v.'ith metamorphosis. 



Metamorphosis has reference to one and the same individual, 

 which leaves the egg under a form different from that of the 

 mother, but in different periods of its life approaches more nearly 

 to this, and, as a full-grown animal, wholly attains it, as is seen 

 in many insects and in frogs. In " change of generation " the 

 mother brings forth a young one that is unlike herself, and re- 

 mains unlike, but from which a progeny proceeds, which cither 

 itself or in its young ones returns to the original form of the 

 mother. Quite unknown the facts were not, on which Steen- 

 strup founded his speculation; but he has the unmistakeable 

 merit of having brought them under a common point of view.- 

 Already had Chamisso, in Saljjce, observed ^proles gr eg aria which 

 alternates with a, proles soUtaria* ; observations on the origin of 

 Cercarice from "yellow worms" had been published by Bojanus 

 and Von Baer ; and the singular propagation of leaf-lice [Aphides] 

 without impregnation, in numerous successive generations during 

 summer, had been made known by C. Bonnet in the last cen- 

 tury. Between Steenstrup and Van Beneden a contest has 

 arisen respecting this theory, which (much to be regretted) has 

 been conducted not without some acrimony. Van Beneden de- 

 sires to substitute the name digenese for Generationswechsel, and 

 regards the essence of the phenomenon to reside, not so much 

 in the form of the body in the different stages of generation as 

 in the propagation by germs and by eggs — an agamic and a sexual 

 propagation. It is not to be denied that Steenstrup did not 

 clearly place this in the foreground; but that it was overlooked 

 by him, I would not venture to assert. At all events, it must 

 be admitted that Van Beneden, though under another name^ 

 has very clearly developed and placed in a fuller light the phe- 

 nomenon to which Steenstrup drew attention, and also in parti- 

 cular has hajjpily applied it to the Cestoids. In regard to the 

 intestinal worms, also, he has had much success in illustrating 

 its true significance. We will here quote a passage to exemplify 

 the lively style of the Louvain Professor, so well adapted to the 

 explanation of the phsenomena : — 



* A. lie Chamisso, De Animalibus quibusdam e classe Vermium. Fasci- 

 culus 1. De Salpa. Berolini, 1819, 4to. I may here notice a slight in- 

 accuracy in the author of the work before us. He speaks fp. 289) of 

 Meyen's observations on Salji<s, and adds, " Lcs biphoirs {Salpte) furent 

 de nouveau etudies peu de temps apres par Chamisso." Meyen's observa- 

 tions were, on the contrary, made fourteen or fifteen years later than those 

 gf Chamisso 



