204 



Biology in Amrrica 



of chromosomes characteristic of the species; which number 

 was thci-cfore restored at the time of fertilization by the 

 union of tlie egg and sperm inielei. Now, if this reduction 

 in number can be shown to involve the separation of definite 



mwmmh 







Photographs of Chromosomes 



Showing various stages in the division of a sea urchin 's egg. The 

 minute dark masses at the center of the egg are the chromosomes. The 

 light areas, surrounded by dark radiations to either side of the chromo- 

 somes are the "asters," so-called from their star-like appearance. 

 Biologists are still in the dark as to the cause of this wonderful process 

 of cell division. In certain respects it closely resem))les an electro- 

 magnetic phenomenon, the poles of the magnet being located at the 

 centers of the asters. These chromosomes are more delicate than the 

 finest filament of a sjjider 's web. Fig. B is magnified 3,000 times, the 

 others 1,500. After Wilson, "Atlas of Fertilization and Karyokinesis 

 of the Ovum. ' ' 



Bi/ pcrminKion of the Macmlllan Company. 



chromosomes from one another so that different sex cells 

 receive different chroinosonie combinations, then an ideal 

 arrangement exists in the cell, for realization of the Men- 

 del ian results. Referring to the case of the tall and dwarf 

 peas, let us suppose that both partners in the cross are "pure," 



