INFUSORIES. 41 



If these animals are not propagated by means of eggs, then 

 neither can they have come into being from eggs, and the general 

 law that all that lives proceeds from an egg (omne vivum ex ovo) 

 does not apply in this case : a wider meaning, then, than the ordi- 

 nary one, must be given to the word egg, which denotes a part 

 that requires impregnation before it can be developed, and contains 

 a yolk with a germ-vesicle surrounded by an external covering. 

 To explain the origin of these creatures in infusions, if equivocal 

 generation be denied, nothing remains but to suppose that the air 

 conveys infusories as it does minute particles of dust, and that the 

 organic matter, which served for the infusion, favoured merely as an 

 abundant nutriment the multiplication of the creatures 1 . The 

 advocates of equivocal generation have not been able to deny the 

 possibility of this explanation: and that possibility destroys the 

 force of all their proofs and argumentations : especially when it is 

 remembered that no space can be so perfectly closed that air cannot 

 penetrate it, and that even boiling does not destroy every kind of 

 infusory : for their opponents themselves could not absolutely deny 

 that infusories were found in boiled infusions which were stopped 2 . 

 Indeed the constancy of form in the species, which had been over- 

 looked by earlier observers in their experiments, or not understood, 

 is irreconcileable with the view that these animal forms are produced 

 by external forces as a mere sport of chance : but it is not by any 



substance assumes a gelatinous form in which the embryos swim, and by which they 

 are surrounded when the cyst is burst. When this substance has been dissolved in 

 the surrounding water they swim freely away, and change the monad form for the 

 vorticelline. STEIN, op. cit. pp. 50 64 and p. 146. Tab. in. fig. 32 41. Also vid. COHN, 

 Zeitschrift fiir Wiss. Zool. in. s. 260 279. Tab. vn. fig. i 12.] 



1 [It is well known that Infusories are conveyed by the air : EHRENBERG found them 

 in the dust borne by the trade- wind : STEIN discovered cysts of Colpoda, of Philodina 

 roseola, and of Macrobiotus Hufelandii, on the terminal twigs of beech-trees that grew 

 2000 feet above the level of the sea. Philodina roseola is the rotifer which gives the red 

 colour to snow. Vid. STEIN, op. cit. p. 25. He quotes EACHENHORST'S assertion that 

 if a slip of glass be moistened by the breath, and moved about in the confined space of 

 an apartment, infusories maybe seen upon it. SCHMIDT'S Jahr-biicher, 1850, Bd. Lxvm. 

 383-] 



3 [The experiments of SCHWANN, POGGENDORFF'S Annalen, Bd. XLI. s. 184, and of 

 HELMHOLTZ, MUELLER'S Archiv, 1843, s. 453, have satisfactorily shown that an infusion 

 boiled so long as to kill any germs previously existing in it, is never visited by infuso- 

 ries if only such atmospheric air be allowed access as has passed through a red-hot tube, 

 or sulphuric acid, or caustic potass.] 



