infusoria: — sexual generation. 463 



circumstances, even after the lapse of several months ; although 

 they were seen in abundance after the free exposure of the same 

 infusions to the atmosphere for a few hours only. Hence it may 

 be fairly inferred, that, as seems to be the case with the Fungi, 

 the dried cysts or germs of Infusoria are everywhere floating about 

 in the air, ready to develope themselves wherever the appropriate 

 conditions are presented ; and all our knowledge of their history, 

 as well as the strong analogy of the Fungi (§ 268), seems further to 

 justify the belief that the same germs may develope themselves 

 into several different forms, according to the nature of the liquid 

 in which they chance to be deposited. — This is a subject peculiarly 

 worthy of the attention of Microscopic observers ; who can scarcely 

 be better employed than in tracing-out the succession of phases 

 which any particular type may present, and in thus making a 

 most important extension of our knowledge of its life-history, 

 whilst at the same time effecting a most desirable reduction in 

 the number of reputed Species. 



347. A very important advance has recently been made in this 

 direction, by the discovery that a true process of Sexual Generation 

 occurs among Infusoria ; — a discovery which has been more or 

 less nearly approached by various observers, but of which the 

 satisfactory completion has been attained by the researches of 

 M. Balbiani. * It appears from his observations, that male and 

 female organs are combined in each individual of the numerous 

 genei*a he has examined, but that the congress of two individuals 

 is necessary for the imjiregnation of the ova, those of each being 

 fertilized by the spermatozoa of the other. The ovarium (or 

 aggregation of germ-cells) is that organ which has been described 

 by many observers as the ' nucleus ;' whilst the testis (or aggrega- 

 tion of sperm -cells) is that which has been described as the 

 ' nucleolus.' The development of each of these organs commences 

 as a single minute cell, which usually multiplies itself in the usual 

 way by subdivision; and when this multiplication has proceeded 

 to a certain point, the cells of the ovary become converted into 

 Ova, whilst those of the testis develope Spermatozoa in their 

 interior. 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 ;f but as we have in the 



* See his "Recherches sur les Phehomenes Sexuels des Infusoires," in 

 Dr. Brown-Sequard's " Jom-nal de la Physiologie " for 1861. An abstract 

 of these researches is contained in the " Quart. Journ. of Microsc. 

 Science," for July and October, 1862. 



t Thus, according to M. Balbiani, the ovary of Chilodon cucullulv.s 

 never advances beyond the condition of a single ' primitive ovum,' 

 formed by the differentiation of the contents of the original ' germ-cell ' 

 into the granular yolk-substance and the pellucid ' germinal vesicle ' im- 

 bedded in it. But in other Infusoria the ' germ-cell ' undergoes repeated 

 subdivisions ; so that from 2 to 4 Ova (as in Paramecium), from 8 to 15 

 (as in Stentor), from 20 to 25 (as in Amphileptus gigas), from 20 to 50 (as in 



