THE CELLULAR BASIS OF HEREDITY 



125 



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 



Fig. 34. Spermatogenesis of a Nematode Worm (AncyracantJius) . A, chromo- 

 somes 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 di- 

 vision after the 5 pairs of chromosomes have pulled apart ; the unpaired chromo- 

 some is going entire to one pole of the spindle ; D, two cells resulting from this di- 

 vision, 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). 



