Throughout the animal kingdom, wherever sexual reproduction occurs, phenomena which can be interpreted as nuclear reduction have been observed in the formation of gametes. In some of the Protozoa this seems to be merely an extrusion of a certain amount of chromatin, but since whenever chromosomes can be observed and counted the process has been found to follow in principle essentially the same lines described above, we have every reason. To believe that it is never a haphazard mass reduction, and that the ripe gametes emerge with a definite chromatin heritage, relatively simple as this may be in the lowest forms.
We have now surveyed the germ cell cycle from fertilized egg through the germ plasm in the adult to the gametes again, but before proceeding to consider the details of the fusion of egg and sperm the fertilization process it may clarify matters to glance back to the chromosome condition in the fertilized egg at the beginning of the cycle which has just been considered. Obviously this fertilized egg (zygote) contained chromosomes, half of which belonged to the egg and therefore may be termed MATERNAL, and half of which were derived from the sperm and thus are PATERNAL.
When the zygote divided by mitosis to form the body and germ, every cell received a set of chromosomes, directly derived from this original set in the zygote. It logically follows, and all observations indicate, that each and every cell, both of the soma and of the germinal tissue, possesses a set of chromosomes, half of which are of maternal and half of paternal origin in other words are direct lineal descendants of the combined set formed at fertilization. So it happens, that each body cell really has a double set (DUPLEX GROUP, DIPLOID NUMBER) of homologous chromosomes and the same is true of the germ cells until maturation. Then at synapsis maternal and paternal chromosomes pair and, after the reduction division, the secondary spermatocytes and oocytes and the gametes themselves have a single set (SIMPLEX GROUP, HAPLOID NUMBER).
Thus far we have emphasized chromatin and, in particular, chromosome reduction as the main purpose of the complicated maturation phenomena. The question now arises: la this chromatin distributed so that all the gametes receive the same heritage?
All the evidence at hand indicates not only that chromosomes differ qualitatively one from another but also that the various parts (CHROMOMERES) of each chromosome are Qualitatively distinct. And further that these qualitative differences are the physical basis of inheritance the determiners (Genes) of characters which will be realized in the individual or the race to which the cell containing them con tributes. Such being the case, the chromosomal complex of the nuclei which arises after synapsis that is, the nuclei Of the gametes depends on how the various chromosomes happen to be distributed during the two maturation divisions.
As a matter of fact all the chromosomal combinations occur which are mathematically possible with the available num ber of chromosomes in a given species, but with one limita tion: every cell must receive one member of each synaptic pair of chromosomes, so that each and every gamete receives a complete simplex group of chromosomes, but rarely the same groups (maternal and paternal) which existed before maturation. For example, if the somatic (diploid) number of chromosomes is eight, sixteen different types of gametes are possible. In Man with 48 somatic chromosomes, and after synapsis 24 pairs of paternal and maternal chromosomes, there are 2 24 , or about twenty million possible types of gametes in each sex, and since these combine at random at fertilization, the number of possible different types of zygotes from one parental pair mounts far up in the trillions. No wonder the children of a family differ there is variation!
In a way, therefore, fertilization is not consummated, so far as its influence on the race is concerned, until the maturation of the gametes in the new generation. We must defer until later the consideration of the significance of these facts in biparental inheritance, and take up now some necessary details of the gametes themselves and of how they unite to form the zygote.