186 BALBIANI, ON SEXUAL 



comes, at length, reduced to this, — that the requisite division 

 of its yolk-mass which, in both, precedes the reproductive 

 act, in the one takes place at a late, in the other at an earlier 

 period. 



To determine, therefore, the exact degree of likeness or 

 unlikeness between the generative organs of any two Infu- 

 soria, it becomes necessary to examine both immediately 

 before the occurrence of fission and of sexual reproduction, as 

 well as in the intervals between these periods. 



Age, too, affects the general aspect of the ovary, more 

 especially in its beaded forms. Still, M. Balbiani thinks 

 that the number of articulations which this organ presents 

 " is almost always in relation with the species or genus of 

 the animal. ^^ 



It follows, also, from what has been said, that any classi- 

 fication of the Infusoria founded on the resemblances or dif- 

 ferences between their reproductive apparatus would be, to 

 a greater or less extent artificial, even in the absence of 

 intermediate modifications to connect together the three 

 leading forms mentioned above. Examples of all these 

 might, without difficulty, be taken from the different mem- 

 bers of the family Trachelina, which can hardly be consi- 

 dered an unnatural group ; or, to confine ourselves to a 

 single genus, Prorodon, we find one species (P. teres), Avith 

 an ovoid nucleus, another (P. niveus), with a cylindrical one. 

 In Spirostonum teres the ovary is undivided ; in S. ambiguum 

 moniliform. 



And in this regard we think that special credit is due to ]M. 

 Balbiani, for the simple clearness with which he has reflected 

 the anatomical facts which lay before him. In the present 

 somewhat confused state of opinion as to what are the most 

 appropriate subdivisions of the Infusoria, how plausibly 

 might he not have sent forth a new arrangement of these 

 animals, based, too, on his own original observations. Let 

 it not, however, be said that the negative result to which he 

 has been led is without its proper value ; for, has he not 

 brought forward a new and striking proof of the doctrine, 

 that 23arts of high import in the vital economy of an organism 

 may, to the pure systematist, be of little value, because their 

 various modifications fail to furnish characters of sufficient 

 definiteuess or constancy ? 



The Act of Sexual Union. 



It has already been stated that the Infusoria, though 

 hermaphrodite, are not self-fertiHzing, in which respect they 



