2 2 GENERAL PHYSIOLOGY 



but the fourth divides once more in each individual, and one 

 half (the " male " nucleus) passes over the protoplasmic bridge 

 into the other individual, so that each one of the pair now con- 

 tains a " female " nucleus of its own, and a " male " nucleus from 

 the other. These two nuclei immediately fuse together and then 

 divide, one half becoming a new macronucleus, and the other half 

 a new micronucleus. After such a mutual exchange of half-nuclei, 

 the pair separate again and the conjugation is ended. 



The phenomena of fertilisation in sexual reproduction are derived 

 phylogenetically from the conjugation of asexual unicellular 



FIG. 83. Fertilisation of the ovum of the thread-worm (Ascaris megdlocephala) in six successive 

 stages. The maturation of the ovum, i.e., the extrusion of the polar bodies, takes place simul- 

 taneously. (After O. Hertwig.) 



organisms ; essentially the same facts are found in the former as 

 in the latter. The process of fertilisation is not entirely the same 

 in different species ; at least in the two species that thus far have 

 been most fully investigated, the egg of the sea-urchin and that of 

 the thread-worm of the horse, some slight differences have been 

 observed, although the essential factors agree throughout. 



We shall consider, first, the fertilisation of the ovum of the thread- 

 worm. The maturation of the ovum, i.e., the extrusion of the 

 polar bodies, takes place while the sperm-cell is entering the egg. 

 While the latter process is taking place (Fig. 83, 7), the egg- 

 nucleus, which up to this time has lain in the middle of the egg> 



