Ill GERM-TRACK 8i 



chromosomes have each broken up into a number of small pieces (Fig. 37, A). 

 The ends of the chromosomes break off as comparatively large club- 

 shaped pieces, and the central portions become divided into a great 

 number of much smaller, more or less spherical, fragments. The larger 

 pieces derived from the ends of the chromosomes are cast out into the 

 cytoplasm, where they degenerate and disappear. For this reason the 

 phenomenon is known as the diminution of the chromatin. 



The cell in which chromatin diminution has taken place is now left 

 with a large number of small chromatin granules, and each of these acts 

 in future mitoses as a single chromosome. The embryo at this stage 

 (prophase of second cleavage division) consists therefore of two cells, 

 the one with two large chromosomes (in A. m. univalens, four in A. m. 

 bivalens), the other with a large number (about sixty in A. m. univalens) 

 of very small ones. The total chromatin content of the latter nucleus 

 is much less than that of the former, owing to the loss of the large pieces 

 from the ends of the original chromosomes. 



All the descendants of the cell which has undergone diminution have 

 the same nuclear composition as their parent cell, the mitosis of the 

 numerous small chromosomes apparently taking place regularly, and 

 there being no further loss of chromatin. It is not so, however, in the 

 case of the cell with the two original chromosomes intact. For three 

 more successive mitoses this cell divides into one which undergoes 

 diminution and one which does not, so that in the i6-cell stage we have 

 one cell with the original chromosomes intact, one in which diminution 

 is in progress and fourteen with the diminished amount of chromatin 

 and numerous small chromosomes. After this, there is no further 

 diminution, so that after the next cleavage there are two cells with the 

 original chromosomes intact, and the remainder with the numerous 

 small chromosomes, and in all future mitoses the daughter nuclei remain 

 of the same composition as their mother nucleus. The two cells with 

 intact chromosomes are the primitive gonad ; the remaining cells will 

 develop into the soma. Thus all the somatic cells of Ascaris megalo- 

 cephala have the numerous small chromosomes and lack the large portion 

 of the chromatin which was thrown out from the ends of the original 

 chromosomes, while all the germ-cells contain the original chromosomes, 

 with all the chromarin, intact (Fig. 38). The Une of cells with intact 

 chromosomes leading from the undivided zygote to the gametes is called 

 the germ-track. 



Phenomena, similar in principle but differing in detail, have been 

 observed in other species of Ascaris. In A. liimhricoides, however 

 (Bonnevie, 1902), only the ends of the chromosomes break off (and are 

 got rid of) ; there is no fragmentation of the middle portions of the 

 chromosomes. In A. canis (Walton, 1918) the chromosome ends are 



G 



