Page:Popular Science Monthly Volume 85.djvu/128

124 of the chromosomes, and there is evidence that one member of each synaptic pair is derived from the father, and the other from the mother. The union of these chromosomes is probably not so close that they lose their identity, though there may possibly be some interchange of substance between them. By this union of the chromosomes into pairs the number of separate chromosomes is reduced to half the normal number; if there are usually 4 chromosomes, as in Ascaris, they are reduced to 2 pairs; if 48 chromosomes, as in man, there are 24 of these pairs.

 A, at the beginning of the growth period showing slender chromatic threads in the nucleus; B, later stage in which these threads ball up and parallel threads conjugate forming the shorter, thicker thread shown in C; D and E, segmentation of the long thread into chromosomes (?) each of which shows its double nature; F, later stage in which the distinctions of the chromosomes is temporarily lost. (After Winiwarter.)

In the conjugation of the chromosomes it is plain that, generally speaking, those chromosomes which are similar in shape and size unite; big chromosomes unite with big ones, little ones with little ones, and those of peculiar shape with others of similar shape (Figs. 34 B, 35 B). It is probable that the two members of a pair of conjugating chromosomes are homologous not merely in shape and size but also in function, though this homology does not amount to identity.

In some instances it can be proved that one member of each conjugating pair of chromosomes comes from one parent and the other from the other parent, and it is probable that this is always true. In every cell of every individual which has developed from a fertilized egg there are two full sets of chromosomes, one of which came from the sperm and the other from the egg; but when this individual in its turn produces