174 GERM-CELL DETERMINANTS [CH. 



BOVERI (1903 a). In the variety univalens of this species 

 there are two long chromosomes in the fertilised egg, each 

 of which has the ends somewhat thickened. In the first 

 segmentation division each of these divides longitudinally 

 in the ordinary way, giving rise to two cells each with two 

 long chromosomes in the nucleus. In the second division, 

 however, the two cells behave differently; in one the two 

 chromosomes divide as before, but in the other they each 

 break up into a string of small elements, and throw off the 

 swollen ends (PL XXI). The small bodies into which the 

 middle part of the chromosomes have broken up divide like 

 ordinary chromosomes ; the cast-off ends do not divide but 

 become included irregularly in the two daughter-cells when 

 the cell divides. In the four-cell stage there are thus two cells 

 with large nuclei containing the complete long chromosomes, 

 and two cells with smaller nuclei in which "chromatin- 

 diminution " has taken place. In the next (third) segmenta- 

 tion division the numerous small chromosomes appear and 

 divide in the two cells that have undergone diminution; of 

 the other two cells, one retains the two full-sized chromo- 

 somes and the other in its turn undergoes diminution, the 

 chromosomes breaking up into numerous small elements 

 and throwing off their ends. In the fourth and fifth divisions 

 the same process is repeated (i.e. four times in all), so that 

 in the equatorial plate of the division between the sixteen 

 and thirty-two cell stages fourteen cells have already the 

 "diminished" chromosomes, in one the chromosomes are 

 undergoing diminution, and in the sixteenth the chromo- 

 somes remain as they were in the fertilised egg. This last 

 cell is the primitive germ-cell; its descendants undergo no 

 further diminution, and from them only the gonads are pro- 

 duced, while the remaining fifteen cells give rise to the rest 

 of the body. 



