232 THE GERM-PLASM 



selves into males and females, and including the innumerable 

 other resulting adaptations and phenomena — take place solely 

 for the purpose of rendering possible the union of the primary 

 constituents of two individuals. 



This process of the fusion of two germ-plasms, which consti- 

 tutes the essential part of fertilisation and is as a rule connected 

 with the fusion of two cell-bodies, I have designated as amphi- 

 mixis. It is not always connected with reproduction, for these 

 two processes take place independently of one another in all 

 unicellular organisms. In the Infusoria, for instance, two indi- 

 viduals in the course of their life-history come into contact with 

 one another, and then either fuse completely into one, or else 

 undergo a partial or temporary fusion ; in both of which cases 

 half the hereditary substance is transferred from one individual 

 into the other, and thus amphimixis is brought about. The 

 latter process is only invariably connected with reproduction in 

 the case of multicellular forms : this is necessitated by the fact 

 that the union of two different germ-plasms cannot take place by 

 the fusion of entire individuals, as the germ-plasm is enclosed in 

 separate cells, a male and a female, the fusion of which takes 

 place in a similar manner to that which occurs in the process of 

 conjugation in unicellular organisms. This act of amphimixis 

 must then be followed by the multiplication of the fertilised egg- 

 cell, accompanied by the differentiation of its successors, — or, 

 in other words, by the 07itogen v of a new individual ; for did this 

 not result, the process of amphimixis would be useless. Amphi- 

 mixis is therefore always connected with reproduction in all mul- 

 ticellular forms, and these tw'o processes together constitute 

 *■ sexual reproduction'' or ' a?nphigony'' (Haeckel). 



The process of amphimixis, as it occurs in amphigonic repro- 

 duction, is briefly as follows. The two kinds of germ-cells 

 mutually attract one another, and then fuse together, the smaller 

 male element always entering the larger female one. The nuclei 

 of the two cells then approach each other, and so come to be 

 situated close together, each being accompanied by its ' centro- 

 some,' — i.e., that remarkable body, enclosed in a clear sphere, 

 which, as already stated, constitutes the apparatus for divisioji. 

 The germ-plasm in both nuclei is at first distributed in the form 

 of fine threads, as is represented in the case of the female nucleus 

 in Fig. 18, I. ; it subsequently, however, becomes contracted, so 

 as to give rise to nuclear rods or ida fits (Fig. 18. II.). Edouard 



