OF THE CELL THEORY 181 



fusion of the male and female pronuclei. Con- 

 cerning the details of this fusion, and especially 

 its relations to the processes of mitosis or karyo- 

 kinesis described above, many elaborate and careful 

 investigations have been made of late years which 

 have brought to light points of extraordinary 

 interest. 



Before describing these in detail it is necessary 

 to say a few words concerning a phenomenon 

 closely connected with fertilisation, though no part 

 of the actual process itself; I mean the extrusion 

 from the egg, prior to fertilisation and independent 

 of it, of the so-called polar bodies. Carus first 

 noticed, in 1828, minute globules on the surface of 

 an egg which took no part in the formation of the 

 embryo. These were described carefully by 

 Friedrich Muller in 1848 as seen by him in the 

 snail's egg, and were called by him directive cor- 

 puscles because of the constant relation they 

 appeared to have to the first plane of segmentation ; 

 he showed clearly that they were derived from the 

 egg itself. Hensen was the first to show that the 

 extrusion of the directive corpuscles or polar bodies 

 occurred independently of fertilisation, for he found 

 that in the rabbit and guinea-pig the polar bodies 

 were formed while the eggs were still in the 

 Graafian follicles, before their discharge from the 

 ovary. This has been confirmed by many other 

 investigators, who have shown that as a rule two 

 polar bodies are extruded in succession ; that both 

 are usually extruded previous to fertilisation ; but 

 that in some cases as in the lamprey and the 



