DEVELOPMENTAL STIMULI IN THE CESTODA. 137 



DEVELOPMENTAL STIMULI IN THE CESTODA. 



To one examining closely the present trend of opinion regard- 

 ing the process which our nomenclature still designates as fer- 

 tilization it is, I think, quite apparent that the evidence and 

 conclusions so logically and convincingly set forth by R. Hert- 

 wig l are gaining a wide acceptance both among special workers 

 and among those who are viewing the data with a somewhat 

 broader perspective. The details of Hertwig's paper had been 

 already accepted by some of the investigators whom he cites, and 

 so far as they concern the Protozoa they are mentioned by 

 Calkins in his recent book as well-established facts, but this 

 cannot detract from the able manner in which Hertvvig has sum- 

 marized these facts and indicated the important conclusions to be 

 drawn from them. The work which to most persons offers con- 

 vincing evidence of the twofold nature of the process we have 

 been calling normal fertilization is the work in " artificial par- 

 thenogenesis " initiated by Loeb and Morgan. One could hardly 

 ask for a more convincing proof that the union of the germ 

 plasms and the "developmental stimulus," as Hertwig calls it, 

 are distinct and separable phenomena although they have become 

 almost indissolubly connected with one another throughout the 

 Metazoa. 



It having been shown in these experiments so far as they have 

 now gone that a stimulus to development may be operative en- 

 tirely independent of any union of the germ-plasms, as for example 

 in the development of eggs of echinoderms or worms upon treat- 

 ment with salt solutions and other stimuli, we should I think seek 

 no less for the converse proposition, viz., tlie union of the genn- 

 plasm and the absence of developmental stimulus (as evidenced by the 

 absence of development) while the oo sperm continues its life. We 

 should then seek for the proper stimulus to this resting condition 

 and by means of this stimulus initiate at will the developmental 

 changes. To illustrate by a hypothetical case, suppose we could 

 have a form where it were possible to bring about at will the union 

 of the ovum and spermatozoon and their nuclei but by subtracting 



1 " Mit welchen Recht unterscheidet man geschlechtliche und ungeschlechtliche 

 Fortpflanzung ?" Sitz. Ber. Gesel. f. Moi'ph. und Physiologic in Mi'uichen, Nov., 

 1899. Translated in Science, N. S., Vol. XII., no. 312, Dec. 21, 1900. 



