188 GENERAL EMBRYOLOGY 



easy, as in the cases previously described, to distinguish at 

 this time between the elements derived from the egg and sperm 

 nuclei. When this duplex nucleus forms its spireme, this 

 segments into the somatic number of chromosomes immedi- 

 ately, and the mitotic figure for the first cleavage forms 

 typically. 



In the formation of the first cleavage figure we see the net 

 result of all the complex processes of the formation and matura- 

 tion of the germ cells, and the union of the two gametes. In a 

 word, what has been accomplished is the reestablishment 

 of a single typical cell with specific organismal characteristics. 

 But this cell now has a nucleus derived in equal parts from two 

 separate individuals of the species. Into this nucleus the 

 events of maturation have made it possible that there should 

 have been brought a complete and equivalent series of chromo- 

 somes from each parent, for the haploid group is composed of 

 one of each pair of chromosomes of the diploid or somatic 

 series. And from this nucleus are derived all of the nuclei of 

 the developing organism; hence every cell of the adult body may, 

 probably does, contain substance derived from both its parents. 



We may regard the organization of these two haploid series 

 into a single nucleus as the culmination of the whole process of 

 fertilization. Or, on the other hand as previously suggested, 

 we may consider the final step in fertilization as not occurring 

 until these pairs of chromosomes actually fuse in synapsis 

 during the maturation of the succeeding generation of germ 

 cells. From this point of view the actual union of maternal 

 and paternal structures and substance never occurs in the 

 somatic cells, for in these synapsis is not known. There is 

 involved in this process of fertilization much more than these 

 simple morphological facts express, and to this subject we 

 shall return presently. 



We shall find it profitable to consider now, as briefly as may be, the 

 phenomena of fertilization and accompanying gamete formation in a 

 series of unicellular organisms of increasing complexity and resemblance 

 to the Metazoa. This subject has been in part postponed from Chapters 

 I and III, and it should be stated again that the series to be described 



