THE GENERAL SIGNIFICANCE OF AMPHIMIXIS 219 



assumed to obtain among them, thus giving rise to the first chromosomes 

 or ids which were complete in themselves. Then came the multipli- 

 cation of these ids V»y the process of division, and only then was the 

 state arrived at from whicli amphimixis, as we now know it, could 

 have arisen, namely, the existence of a considerable number of identical 

 ids, half of which could be exchanged for the identical ids of another 

 individual in conjugation. 



But as to our question, In what organisms did amphimixis first 

 arise, and how ? there seems, from what we have already learned 

 Avith regard to the Coccidia, little prospect of our being able to give a 

 definite answer, for if amphimixis occurs even in these lowly organisms, 

 and occurs, too, in the same manner as in the higher unicellular 

 organisms, and not very much more simply than among the highest 

 multicellular organisms, we may conclude that the preliminary stages 

 will now be very difficult or impossible to detect, either because they 

 are extinct, or because they occur only in ultra-microscopic organisms. 



Nevertheless there do appear to be preliminary stages, and they 

 are exactly those which we should have assumed if we had been 

 obliged to construct them theoretically. 



The first phenomenon of this kind is the mere juxtaposition of 

 two or more unicellular organisms, without the occurrence of fusion. 

 This was probably first observed by Gruber in Amoeba3, and it was 

 theoretically interpreted at a later date by Rhumbler. As many as fifty 

 Amoebfti gather together to form a ' nest,' and remain closely apposed 

 to each other for a fortnight. Although no fusion took place, and there 

 were no visible results of this juxtaposition, it may be concluded that 

 the animals had some sort of attractive effect upon each other, and it 

 may be supposed that some sort of advantage must have been 

 associated with this state of quiet, close apposition against one another. 

 Cytotropism, the mutual attraction of similar cells, which Wilhelni 

 Roux first observed in the segmentation-cells of the frog's q^^, seems 

 to occur also in unicellular organisms, and this may help us to under- 

 stand how a fusion of cell-bodies may have come about. 



Fusion of this kind was demonstrated in the Myxomycetes 

 almost forty years ago by De Bar}^ and it has been observed more 

 recently in various unicellular organisms, especially in Rhizopods and 

 in Heliozoa. These last often place themselves close together in pairs, 

 threes, or even more at a time, and then the delicate cell-bodies 

 coalesce, though no fusion of the nuclei takes place. With Hartog, 

 we call this process ' Plastogamy,' but we cannot agree with that 

 observer when he regards the importance of the process as con- 

 sisting in the fact that the nuclei thus come into contact with fresh 



