AMPHIMIXIS AS A CONDITION OF LIFE 25 



poucli whicli used to serve for the reception of the spermatozoa 

 still persists, though it is invariably empty. 



In what sense, then, it may be asked, can amphimixis be 

 regarded as a condition of the continuation of life ? The reply 

 is that, in the case of those species which reproduce by amphi- 

 mixis, the conditions of reproduction are such that the germ- 

 cells of the two sexes are incapable of development by them- 

 selves, and that amphimixis, by bringing about a fusion of these 

 elements, permits their subsequent development. We have 

 already noticed how the egg and the sperm-cell complete each 

 other. The egg, in order to be ripe for fertiUsation, must pass 

 through certain changes. These include the loss — or, at all 

 events, the degeneration — of the centrosome, and the entrance 

 of the spermatozoon in amphimixis can alone remedy the de- 

 ficiency and enable the egg to develop. Here, therefore, amphi- 

 mixis appears as an indispensable condition for the continuance 

 of the life of the species. But even this is not absolutely true. 

 The egg can obviate the apparent necessity of amphimixis by 

 undergoing only a part of the changes involved in maturation. 

 This is well illustrated in artificial parthenogenesis. 



The remarkable experiments of TichomirofE and Loeb have 

 shown that, when certain ova are subjected for a short time to 

 the influence of sulphuric acid or to sea-water with some mag- 

 nesium chloride added to it, parthenogenetic development may 

 ensue. It seems as if the degeneration of the centrosome of the 

 egg can be thus counteracted. In short, the influence of various 

 kinds of chemical reagents artificially introduced may have the 

 same result as amphimixis, as far as the division of the egg and 

 the development of a larva are concerned.^ 



^ The observations of Petrunkewitscli have confirmed the supposition 

 that the effect of artificial parthenogenesis in the sea-urchin is to pre- 

 vent the dissolution of the centrosome of the egg. It is not a new centro- 

 some which is formed, but the original persists, in a sense, " strengthened " 

 by the aid of the artificially-introduced salts {vide report in Zoologische 

 Jahrhiicher, Supplementsband vii., Jena, 1904). 



