MITOSIS IN CHICK TISSUE CULTURES. 157 



of enumeration of the chromosomes it is quite satisfactory. 

 Centrosomes and spindles are quite similar in cells dividing 

 under the two conditions although they are clearer and possibly 

 larger in the cultures. 



An examination of the chromosome number has developed no 

 distinctions between the two types of material. In the prophase 

 of the culture cells the number of chromatic individuals is very 

 large (from 60 to 70 in some cases). Prophases in which all 

 chromatin rods or bodies can be counted are somewhat rare in 

 sectioned embryonic cells but enough have been observed to 

 indicate their entire similarity to the former. As the metaphase 

 stage is approached the number of distinct chromosomes becomes 

 less, presumably through the coalescence of certain chromatin 

 granules or rods which previously had seemed to be separate. 

 (See Fig. 2 for various series of chromating granules lying in 

 straight lines which probably coalesce later in the mitotic process 

 to form single chromosomes.) This results in a metaphase 

 chromosome number that probably is between 35 and 40, it 

 being difficult to determine the exact number owing to the 

 minuteness of the smaller chromosomes. Nothing unusual de- 

 velops in the culture anaphases and division is completed as 

 normally there as in the embryo. 



The tissue culture divisions were originally called to my 

 attention by Mrs. Lewis because the chromosomes seemed to show 

 some evidence of fragmentation such as I had reported rinding in 

 other forms (Hance, '17 and '18). Certain cells did possess a 

 larger number of chromosomes than others. As already stated 

 above the larger number is apparently an early prophase condition 

 and the number decreases as the cell approaches metaphase 

 through the union of what, in the prophase, have seemed to be 

 discrete bodies. This condition occurs in embryos as well as in 

 cultures. There is a little evidence drawn from instances where 

 occasional somatic chromosomes are apparently without mates 

 identical in form and size that suggests the possibility of fragmen- 

 tation. However, considering the quite definitely determined 

 condition discussed above of the running together of prophase 

 granules to form chromosomes the asymmetry of the mates may 

 as well, or perhaps even more likely, be due to the non-completion 

 or total failure of the coalescing process as to fragmentation. 



