66 



PROBLEMS OF FERTILIZATION 



These are the only components of the germ nuclei that 

 can be traced morphologically beyond this time. Even 

 in those cases in which a typical first segmentation 

 nucleus occurs (Fig. p) it is equally certain that the 

 maternal and paternal chromatins form equal chromo- 



a 



&t> 



%^ 



Fig. g. — (After Boveri): a, Pronuclei of Ascaris megalocephala, 

 approaching between the attraction spheres of the first cleavage spindle, 

 each containing two chromosomes, b, The first cleavage spindle fully 

 formed; it contains four chromosomes which are shown in a polar view of 

 the same spindle in the small figure to the right above. Two of these 

 chromosomes are of maternal and two of paternal origin, c, Meeting 

 of germ nuclei of Pterotrachea (pteropod) ; each contains sixteen chromo- 

 somes, d, Formation of the first cleavage spindle in Pterotrachea. The 

 maternal and paternal chromosome groups are separate. 



