184 ROBERT T. HANCE 



broken off and that the loss is Hmited to that amount of chroma- 

 tin. Since the fragments are fairly uniform in length, the 

 chromosomes must be reduced by more or less equivalent 

 amounts, and consequently we should not expect to find the 

 percentage relation between the pairs showing any marked 

 variation from that found between the spermatogonial pairs. 

 This is shown graphically in text figure 4 where the percentage 

 relations of the first twenty pairs of four highly fragmented cells 

 are plotted. As can be seen, the curves vary but slightly from 

 the spermatogonial curve. 



It has been pointed out that the long chromosomes are the 

 chief ones involved in fragmentation. A study of the longer 

 chromosomes Plate 10, rows 1 and 2 (unfragmented cells) reveals 

 that several are J-shaped or formed as an unequal armed V. 

 Assuming that the point of spindle fiber attachment is located 

 at the apex of the J (page 163) the possibility that fragmenta- 

 tion has occurred at this point immediately suggests itself since 

 the short arm of the chromosome is in many cases approximately 

 the length of the chromosomes (the fragments) lying above 40 

 in the fragmented series. It will be noted on Plate 10 that 

 there are several J-shaped chromosomes in those rows represent- 

 ing cells in which little or no fragmentation has occurred. With 

 an increasing degree of fragmentation the number of curved 

 chromosomes becomes less and less. In cells with high num- 

 bers of chromosomes those lying in the same columns as the 

 curved chromosomes of rows 1 and 2 are generally rod-shaped. 

 While this is but circumstantial evidence it offers a logical and 

 interesting explanation of the process as occurring in these cells. 

 It is, of course, but a tentative theory which may be proved 

 or overthrown by further work. 



DISCUSSION 



The work of other mammalian cytologists 



The amount of published cytological work on the mammals 

 is neither large nor reliable, when compared to that which has 

 been done on other forms as, for instance, the grasshoppers and 



