202 



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. S3, — Fertilisation of the ovum of the thread-worm (Jscctj-zs iJiejya/ocep/ia^a) in six successive 

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

 taneouslji. (After O. Hertwlg.) 



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, /), the egg- 

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



