MICROSCOPIC FORMS OF ANIMAL LIFE. 51 



the ova, those of each being fertilized by the spermatozoa of the other. 

 He regards the ' nucleus ' as an ovarium or aggregation of germs, 

 whilst the ' nucleolus ' is really a testis or aggregation of spermatozoids. 

 The particular form and position which these organs present, and the 

 nature of the changes which they undergo, vary in the several types of 

 Infusoria; but as we have in the common Paramedum aurelia an exam- 

 ple, which, although exceptional in some particulars, affords peculiar 

 facilities for the observation of the process, and has been most com- 

 pletely studied by M. Balbiani, it is here selected for illustration. This 

 Animalcule, as is well known, multiplies itself with great rapidity (under 

 favorable circumstances) by duplicative subdivision, which always takes 

 place in the transverse direction; and the condition represented in Plate 

 xiv., Figs. 1, 2, is not, as has been usually supposed, another form of 

 the same process, but is really the sexual congress of two individuals, 

 previously distinct. When the period arrives at which the Parameda are 

 to propagate in this manner, they are seen assembling upon certain parts 

 of the vessel, either towards the bottom or on the walls; and they are 

 soon found coupled in pairs, closely adherent to each other, with their 

 similar extremities turned in the same direction, and their two mouths 

 closely applied to one another, but still continuing to move freely in the 

 liquid, turning constantly round upon their axes. This conjugation 

 lasts for five or six days, during which period very important changes 

 take place in the condition of the reproductive organs. In order to dis- 

 tinguish these, the Animalcules should be slightly flattened by compres- 

 sion, and treated with acetic acid, which brings the reproductive 

 apparatus into more distinct view, as shown in Figs. 1-5. In Fig. 1, 

 each individual contains an ovarium a, which is shown to present in the 

 first instance a smooth surface; and from this there proceeds an excretory 

 canal or oviduct c, that opens externally at about the middle- of the 

 length of the body into the buccal fissure e. Each individual also contains 

 a seminal capsule #, in which is seen lying a bundle of spermatozoids 

 curved upon itself, and which communicates by an elongated neck with 

 the orifice of the excretory canal. The successive stages by which the 

 seminal capsule arrives at this condition, from that of a simple cell, 

 whose granular contents resolve themselves (as it were) into a bundle of 

 filaments, are shown in Figs. 6-10. In Fig. 2, the surface of the ovary 

 , is seen to present a lobulated appearance, which is occasioned by the 

 commencement of its resolution into separate ova; while the seminal 

 capsule is found to have undergone division into two or four secondary 

 capsules b, b, each -of which contains a bundle of spermatozoa now 

 straightened out. This division takes place by the elongation of 

 the capsule into the form represented in Fig. 11, and by the narrowing 

 of the central portion whilst the'extremities enlarge; the further multi- 

 plication being effected by the repetition of the same process of elonga- 

 tion and fission. In Fig. 3, which represents one of the individuals still 

 in conjugation, the four seminal capsules Z>, #, are represented as thus 

 elongated in preparation for another subdivision, whilst the ovary , , 

 has begun, as it were, to unroll itself, and to break up into fragments 

 which are connected by the tube m. It is in this condition that the 

 object of the conjugation appears to be effected, by the passage of the 

 seminal capsules of each individual, previously to their complete matura- 

 tion, into the body of the other. In Fig. 4 is shown the condition of a 

 Paramedum ten hours after the conclusion of the conjugation; the 

 ovary has here completely broken up into separate granular masses, of 



