468 H. L. WIEMAN 



in the diploid number by treating developing copepod eggs with 

 ether; while Nemec has caused their formation in plant tissue by 

 means of chloral hydrate. 



Agar in his study of the chromosomes in larval Lepidosiren 

 sums up his conclusions regarding this form of chromosome 

 in these words: "The tendency for chromosomes to become 

 transversely segmented or constricted is a wide-spread charac- 

 teristic. It becomes especially operative, but not solely, whenever 

 the chromosomes are short in comparison with their length as 

 happens normally in meiosis and exceptionally in somatic tissue" 

 (p. 295). 



One may readily find stages illustrating the steps in such a 

 process. Figure 10 is an enlarged drawing of the chromosomes 

 a,b,c,d and e of figure 3, which show how the segmentation might 

 be brought about; a may be taken as the first step, the reduction 

 in length resulting in a concentration of chromatic material at 

 either end with a thinning out of the middle region. This thining 

 out which gives the appearance of a transverse segmentation does 

 not always occur in the middle, as may be seen from b. In c 

 the thin, or more lightly staining region is slightly constricted, and 

 in s and e the constriction is well marked. The constriction does 

 not represent a line of future division, for it can be clearly demon- 

 strated that division takes place at right angles to it, that is, 

 in a plane passing through the long axis of the chromosome. It is 

 when such a segmented chromosome is beginning to divide that 

 the tetrad-form is produced. 



DISCUSSION AND CONCLUSIONS 



In the foregoing I have described somatic mitoses in which 

 thirty-three, thirty-four and thirty-eight chromosomes occur. 

 In addition to these I have observed other cases in which the num- 

 ber is thirty-four. In still other instances it is impossible to state 

 the number with certainty, but careful examination of several 

 scores of mitoses leads me to believe that the number thirty-four 

 approximates the one that occurs most frequently in the cells of 

 the embryo under consideration. 



