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POPULAR SCIENCE REVIEW. 
tinct sexual organs have been found, and ova resulting from 
their conjugation have been traced through a regular develop- 
ment. Contemporaneously with or between the periods of 
this development another and distinct set of changes known 
as cc metamorphosis,” almost invariably occur, the phases of this 
metamorphosis being more or less marked. But that neither 
metamorphosis nor gemmiparous reproduction ensures the 
perpetuation of species we have the strongest evidence even 
in the very lowest forms of animal life — the infusorial Protozoa. 
Siebold first discovered that the so-called nucleus inside the 
body of these creatures was an egg-producing organ, a fact 
confirmed by Stein, who described the breaking up of the 
nucleus and the process of encystation of the vorticellariee ; 
and by Cohn, who observed similar generative phenomena and 
metamorphoses in a great number of other infusoria. Car- 
penter also early expressed the opinion that something of the 
nature of sexual reproduction might be discovered to take 
place in these animals.* But it was reserved for Balbiani, 
who had long studied the phenomena of infusorial generation, 
to place beyond doubt the actual facts, which he summed up 
in his published memoir, as follows : — 
1. The infusoria are no exceptions to. the general laws that govern the 
reproduction of organized beings. 
2. They possess complete hermaphrodite sexual organs, but two individuals 
are necessary for conjugation and fecundation. 
3. The conjugation is effected by simple apposition of the ventral surfaces, 
on which occasionally an aperture is observable. 
4. This condition (conjugation) is that which is usually described as longi- 
tudinal fission. The only exceptions to this rule are the vorticellariae. 
5. The organs of reproduction are the nucleus and nucleolus, the former 
being the female, the latter the male organ. 
6. Each of these organs appears at first under the form of a simple cell 
(primitive ovule), which produces by consecutive transverse fission 
other similar cells, which again either become ova, or develop within 
their interior spermatozooids. 
7. The development from the primitive ovules (male and female) is per- 
fectly analogous. 
* Muller, Lachman, and Clarapede had also observed the spermatozooids 
within a cell, and Lieberkiihn in a nucleolus. As we commenced this article 
with a confession that the newest discovery was but a return to the idea of 
the oldest discoverers, we must, to be consistent with ourselves and just to 
the great historian of the Infusoria — Ehrenberg, — remind our readers that 
this interpretation of nucleus and nucleolus was first glanced at by him ; 
inasmuch as he termed the nucleus a “ sperm-gland,” and consequently that 
the proof only, not the idea, is new. 
