144 EZRA ALLEN 



would be indistinguishable, as their size is so nearly equal. 

 Chromosome E has been seen in many cells. Whether its form 

 is constant, or whether it may assume the form shown in figure 

 28, is a matter that has not yet been determined. 



The other groups of rings of medium and of small size are not 

 to be followed individually for similar reasons. The two large 

 crosses are in the same category. The accessory, however, 

 always stands out distinctly. Its history is treated separately 

 on p. 151. 



The constancy of form of all, or of the principal chromosomes, 

 is too large a topic for complete discussion in this paper. Aside 

 from the evidence cited for A and B, my observations lead me to 

 conclude that such constancy is doubtful. Numerical data of 

 large numbers are necessary to establish this point. Whatever 

 such study may show, my observations upon hundreds of cells 

 leads to the conclusion that the shapes assumed are limited to the 

 various types of ring (as seen in figures 27, 28 and 31), the cross, 

 and, in the case of the accessory only, the curved rod. 



The composition of the first spermatocycte euchromosomes. 

 This topic is closely related to the former in that it treats of the 

 organization of the shapes just described.' Briefly stated, this 

 organization is typically tetrad, with the sole exception of the 

 accessory, which is a diad, and is treated elsewhere as a hetero- 

 chromosome (p. 151). By ' typically tetrad ' is meant that there 

 are four elements in each chromosome, each of which is the pro- 

 duct of the longitudinal splitting of the respective paternal and 

 maternal chromosomes which paired in synapsis. McClung's 

 name for these elements, 'chromatid,' will be used in this paper. 

 His interpretation of a chromosome as the unit'mass of chromatin 

 which divides in cell division is also retained in the discussion of 

 the spermatocyte euchromosomes, although some writers have 

 preferred to think of these chromosomes as pairs of chromosomes 

 and so to denominate them. 



The tetrad composition is clearly seen in the large and medium- 

 sized chromosomes. In the case of the simple rings it appears 

 clearest in the midprophase conditions (fig. 27), but it is to be 

 seen in the large double ring in diakinesis (fig. 34) . These larger 



