56 HISTORY OF THE HUMAN BODY 



others become yolkless, abortive, eggs, attached to the ovum 

 and called polar globules. Owing to the enormous disparity 

 in size between the abortive and the functional ova the divi- 

 sions of the oogonium by which they are formed were for a 

 long time not recognized as true cell divisions, but the polar 

 globules were spoken of as extruded or cast off from the 



V VI VII 



FIG. ii. Diagram of fertilization. 



Stage I represents the egg just previous to maturation. The chromosomes, ar- 

 ranged in tetrads, are twice the number found in somatic cells, which, in this dia- 

 gram is assumed to be 12. At II a mitotic figure is formed, which, in III, results 

 in the formation of two cells; a little one, the first polar globule, a, and the egg, 

 each with a reduced number of chromosomes, in this case 12. In IV and V a 

 second mitotic figure is formed, which results in the expulsion of a second polar 

 globule, b, and the reduction of the chromosomes of the egg nucleus to six, one- 

 half the normal number. Meanwhile a spermatozoon head has entered the egg, com- 

 posed mainly of chromatin, the equivalent of the six chromosomes of the reduced egg 

 nucleus, and a new centrosome, to replace that of the egg which was destroyed during 

 the expulsion of the second polar globule. The spermatozoon head rotates through 

 180, thus bringing the centrosome between the male and female germ nuclei, as 

 in VI. The first cleavage spindle is seen forming in VII and VIII, after which 

 the cell divides into two and development begins. 



" egg," terms which express the phenomena as observed, but 

 mask their true biological significance. 



Usually, owing probably to the rudimentary condition of 

 the polar globules and their lack of function, the globule 

 formed by the first of the two reductive divisions, and hence 

 the equivalent of the definite ovum plus one abortive egg, 

 does not carry through its division into two, but remains as 



