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segmentation ; many of the bodies in which they occur being 

 ova or germs at some period of development. 



As far as we are yet acquainted, all permanent animal forms 

 propagate by ova, and this seems to be their starting zoological 

 point. It is true that reproduction by scission and by buds is 

 not uncommon among the lower invertebrates, but even where 

 these last modes are most common, they move only within a 

 certain cycle, and after a longer or shorter period, it seems 

 necessary for the complete animal to begin a new round in gene- 

 ration by the union of the sexual products, the ovum and the 

 spermatic particle. This scission and budding seem to be only 

 the extension of the active germ-power under individual forms, 

 and can never, it would seem, be indefinite ; for in the virgin- 

 reproduction of the Aphides or plant-lice, continued, as has 

 been sometimes observed, through several years, there comes a 

 time at last when the cycle is finished, and reproduction, by 

 distinct sexes, preserves the race by beginning a new series. 

 This law in Zoology, the necessity of some period of true 

 sexual generation, seems so rigid, that from it alone one would 

 be almost justified in presuming that those infusorial organisms, 

 which are evidently too small to admit the existence of distinct 

 sexual organs, ovaria and testes, must, therefore, be regarded 

 out of the proper domain of Zoology. I say too small, in the 

 above passage, for all reproductive sexual processes are invaria- 

 bly carried on through the medium of cells, and nucleated cells 

 have a point of origin far beyond or above such minute forms as 

 must here be present,if at all ; and no doctrine is more untenable 

 than that which declares that organic, functional particles have 

 no definite limit as to minuteness ; for there is an observable con- 

 fine on which life and matter seem to meet, as it were, for the 

 first time, and below which all is lifeless and confused. To as- 

 sert, then, that these infusorial organisms do, or even can have, 

 distinct sexual organs, is as gratuitous as it is unphysiological. 



This subject may be partially illustrated by alluding, briefly, 

 to some studies I have recently prosecuted on infusorial forms, 

 and especially some of the parasitic ones I have found in the 

 intestinal canal of ants. By examining very many specimens, I 

 have been enabled to work out, pretty definitely, their character- 

 istics and developments. Under some circumstances they are 



