THE MORPHOLOGY OF FERTILIZATION 67 



some groups upon the first segmentation spindle, 

 because they are usually slightly separated and it is 

 known that each germ nucleus contains only the hap- 

 loid number of chromosomes, whereas the first segmen- 

 tation spindle has always the diploid number. 



Van Beneden (1883) was the first to discover this 

 invariable law of fertilization; he discovered that in 

 Ascaris megaloccphala each germ nucleus produces two 

 chromosomes, so that the first segmentation spindle 

 contains four (Fig. 9). These divide longitudinally in 

 the usual way so that the first two cells each receive 

 four daughter-chromosomes, two of maternal origin 

 and two of paternal origin; he assumed that this condi- 

 tion was transmitted to all subsequent generations of 

 cells, which thus possess nuclei of biparental origin. 

 This involves the idea that maternal and paternal chro- 

 matins remain distinct within each cell throughout the 

 life-history, and thus a basis is furnished for explaining 

 both the intimate intermingling of paternal character- 

 istics in the offspring and the independent behavior 

 of such characteristics in heredity. Subsequent studies 

 of the behavior of the germ nuclei include a large num- 

 ber of forms and have demonstrated the same principles 

 to be universal. 



We must dwell further upon this question. Un- 

 doubtedly the most fundamental fact which the mor- 

 phological study of fertilization has revealed is the 

 equivalence of the germ nuclei with reference to the 

 chromosomes. This is a definite and undeniable posi- 

 tive result which is in perfect agreement with the equiva- 

 lence of the sexes in inheritance. In all other respects 

 the germ cells are differentiated in opposite directions; 



