442 M. Balbiani on the Generative Organs of the Infusoria. 



whole of the formative sac. Although I have never seen them 

 execute any movements, I do not hesitate in considering them as 

 the spermatic filaments of these animals. 



IV. It is with equal certainty that we may call the nucleus the 

 female genital organ of the Infusoria, in opposition to the per- 

 fectly hypothetical assertion of Ehrenberg, who regards it as the 

 testicle. Its evolution likewise only commences at the time of 

 reproduction, and often during the sexual union itself. In P. 

 Aurelia and caudatum, towards the end of the copulation, its sur- 

 face is traversed in all directions by numerous furrows, which, 

 penetrating deeper and deeper into its mass, finally divide it 

 into a great number of unequal and irregularly rounded frag- 

 ments, having a clear centre more or less surrounded by granules. 

 I should compare these with the first rudiment of a vitellus, and 

 the transparent central portion to a more or less developed ger- 

 minal vesicle. The fragments thus formed are soon dispersed 

 in the surrounding parenchyma. Here a very small number of 

 them, almost always four, never more and very rarely less, com- 

 plete their evolution, and soon acquire the appearance of com- 

 plete and well- developed ova. In this state they present them- 

 selves in the form of small brilliant bodies, perfectly equal in 

 volume, slightly oval, and of a bluish-grey appearance. We may 

 very clearly distinguish in them a finely granular vitellus, sur- 

 rounded by its proper membrane, which separates from it more 

 or less after a few moments' exposure to water. The germinal 

 vesicle and spot are also visible with a distinctness truly sur- 

 prising, considering that we have to do here with the smallest 

 of living organisms. I have met with these ova still enclosed in 

 the body of the animal on the seventh day after the copulation ; 

 they no longer exhibited either germinal vesicle or spot, and 

 their volume had slightly increased. In the allied species, P. 

 Bursaria, the reniform nucleus becomes unrolled before breaking 

 up, and in this state resembles the ribbon-shaped nucleus of 

 the Vorticellce. About twenty or twenty-five of the fragments 

 produced from it continue their development and become so 

 many perfect ova. In the nucleus of Chilodon cucullulus, also, 

 we observe, after the copulation, the disappearance of the trans- 

 parent zone with its central obscure spot. In the genera Stylo- 

 nychia and Urostyla the ova are four in number, as in Parame- 

 cium caudatum, but they are produced by a different mechanism. 

 Each of the two nuclei divides into two halves, as in the act of 

 spontaneous division ; and the four fragments thus produced form 

 an equal number of perfect ova. Lastly, in Spirostomum ambi- 

 guum, we have seen, in individuals which have been copulating 

 for some time, the forty or fifty grains of the long flexuous cord 

 which traverses the body become rounded and detached from 



