Page:Popular Science Monthly Volume 85.djvu/129

Rh germ cells homologous chromosomes of each set unite in pairs during the growth period.

These synaptic pairs are the bivalent chromosomes, and in addition to showing the line of junction by which they are united they frequently show a longitudinal split through the middle of each chromosome and

 (Ancyracanthus). A, chromosomes of sperm mother cell, 11 in number, before their union into pairs; B, early stage of first division; 10 of the chromosomes have united into 5 pairs and each of these has split lengthwise; 1 chromosome remains unpaired; C, first maturation division after the 5 pairs of chromosomes have pulled apart; the unpaired chromosome is going entire to one pole of the spindle; D, two cells resulting from this division, one containing 5 and the other 6 chromosomes; E, four cells resulting from the division of two cells like B, in which each chromosome has split into two so that changing into spermatozoa, one containing 5 and the other 6 chromosomes. (After two of the cells contain 5 and two contain 6 chromosomes; F, two of these cells Mulsow.)

at right angles to the line of junction. It thus happens that these bivalent chromosomes are frequently four-parted and such four-parted chromosomes are known as tetrads (Figs. 34 B, 35 B).