THE CELLULAR BASIS 141 



osomes, and in addition to showing the line of 

 junction by which they are united they fre- 

 quently show a longitudinal split through the 

 middle of each chromosome and at right angles 

 to the line of junction. It thus happens that 

 these bivalent chromosomes are frequently 

 four-parted and such four-parted chromo- 

 somes are known as tetrads (Figs. 32 B, 

 33 B). 



(c) The Maturation Period. — Finally at 

 the close of the growth period both oocyte and 

 spermatocyte undergo two peculiar divisions, 

 one following immediately after the other, 

 which are unlike any other cell divisions. 

 These are known as the first and second mat- 



Fig. 33. Oogenesis of a Nematode Worm (Ancyracanthus) . 

 A, egg mother cell containing 12 chromosomes before their 

 union into pairs. B, early stage of first maturation division; 

 all the chromosomes have united into 6 pairs, and all but one 

 of these has split in two so that the pairs are really four-parted 

 (tetrads). C, the six tetrads in the first maturation division. 

 D, egg containing 6 chromosomes, after both first and second 

 maturation divisions; the eliminated chromosomes are shown 

 as the polar bodies at the margin of the egg. E and F, eggs 

 after fertilization; the egg nucleus is above and contains 6 

 chromosomes, the sperm nucleus is below and contains 5 chro- 

 mosomes in one case and 6 in the other; in the former case the 

 egg becomes a male with 11 chromosomes, in the latter a female 

 with 12 chromosomes. (After Mulsow.) 



