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 
Jaw 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!. 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?. 
Indeed the constancy of form in the species, which had been over- 
looked by earlier observers in their experiments, or not understood, 
+s jrreconcileable 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. 111. fig. 32—41. Also vid. CoHN, 
Zeitschrift fiir Wiss. Zool. Il. s. 260—279. Tab. Vil. 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 
roscola, and of Macrobiotus Hufelandii, on the terminal twigs of beech-trees that grew 
2000 feet above the level of the sea. Philodina roscola is the rotifer which gives the red 
colour to snow. Vid. STEIN, op. cit. p. 25. He quotes RACHENHORSI’S assertion that 
if a slip of glass be moistened by the breath, and moved about in the confined space of 
an apartment, infusories may be seen upon it. ScHMIDT’S Jahr-biicher, 1850, Bd, LXVIII. 
8. 383-] 
2 [The experiments of ScHWANN, PogcEnporFF’s Annalen, Bd, xr. s. 184, and of 
Heimuoirz, MuELLER’s Archiv, 1843, 8. 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. | 
