STUDIES OP REPRODUCTIVE ELEMENTS: I. 21 



transversely. This stage corresponrls in the egg to the formation of 

 the first polar hody, which takes place exactly in the same manner as 

 that of the first division of the sperm-mother cell. En hoth cases the 

 original eio-ht chromosomes hecome divided into two, giving rise to 

 the dauirhter cells also coiitaininii* eiii'ht clu-oniosomos (Wn^waim^^ 

 " Eqiiationstheilnng "). 



5. This is immediately followed ])y another division, witliont 

 an intervening resting stage of the rniclei, and 1)y reduction in the 

 original number of chromosomes (^Weismann\s " Redulctionstheilung '*). 

 The eight chromosomes which at first are arranged in a single row 

 become arranged in double rows of tour each, and the cell begins to 

 divide in such a manner that four chromosomes go to one cell and the 

 remaininii' four to the other. 



]')y divisions described under Xo. 4 and 5 u s])erm-mother cell as 

 well as an egg-mother cell increases to: four cells, which in case of the 

 former give rise to four spermatozoa, while in tliat of the latter a single 

 egg cell with two polar bodies, results (or tlu'ce when the first polar 

 body divides). 



6. During the formation of the second polar 1)ody the spermatic 

 cell enters into the egg-cell. The nucleus of the sperm cell is, as a 

 general tlu'ng, at first rather small and ('(flours deeply and homogeneous- 

 ly, but soon the diiferentiation sets in and the four chromosomes become 

 distinctly visil)le. The nucleus of the egg-cell remaining after the 

 formation of the secoijd polar Ijody — tlie female pronucleus of Ed. van 

 Beneden — shows at first distinctly four elements. These soon grow 

 longer, show many convolutions, and pass into a " skein " stage. 



7. The two nuclei gradually approach one another until they 

 come in close contact, but they do not unite into a single piece before 

 the equatorial plate is formed. The number of chromatic elements in 

 each of the copulating nuclei is now found to be eight, exactly double 



