108 bulletin: museum of comparative zoology. 



diameter varies at different points and exceeds 0.83 micra. (3) The 

 rods are indivisible units, and, since each spermatogonial and second 

 spermatocyte chromosome is composed of two, and each primary 

 spermatocyte chromosome of four, their morphological identity is 

 metrically proved. (4) The eight rod-lengths are not the same in 

 any two species; the longest and 5 short chromosomes occur in all, 

 but identity is always established by the two remaining chromosome- 

 rods. (5) The complexes of a species and its variety appear to be 

 identical; differences if existing, are too small to be recognized. (6) 

 The somatic chromosomes are identical with those of the germ cells. 

 (7) The total volume of ordinary chromosomes is the same in sperma- 

 togonial and primary spermatocyte metaphases, whereas only half this 

 amount appears in that of the secondary spermatocyte." 



For some zoologists, the fact that for any species the chromosomes 

 reappear in the different cell-generations possessing the same relations 

 as to number, shape, and size is merely an expression of the activities 

 of the cell and signifies nothing as to a persistent individuality of the 

 chromosomes. Fortunately, we are not dependent on this kind of 

 evidence alone, the results of studies of the chromosomes of hybrids, 

 for example, offering still stronger evidence of individuality, as shown 

 by the work of Moenkliaus on hybrids between Fundulus and Men- 

 idia, and that on echinoderm hybrids by Baltzer and by Tenent. 

 Moenkhaus ('04) could recognize the two sets of chromosomes arising 

 from the pronuclei of the diverse parents by characteristic differences 

 in size. For the first two or three cleavages the two groups tended to 

 remain distinct, but in later cleavages the chromosomes of the two 

 kinds become more and more intermingled, though they are still 

 recognizable by their characteristic sizes. The value of this evidence 

 is obvious, and on this point Moenkhaus says (p. 53) : — "As long as 

 the two kinds remain grouped, as during the first two divisions, this 

 fact has little added significance (i. c, that two groups of distinctly 

 different kinds of chromosomes arise), since within each group it 

 would be perfectly possible for the component chromosomes to ex- 

 change chromatin granules during the resting period. If, however, 

 as occurs in the later cleavages, the two kinds of chromosomes become 

 mingled, the chromatin granules of both kinds must lie mingled to- 

 gether within the resting nucleus. If from such a nucleus the two 

 kinds of chromosomes again emerge, it amounts almost to a demon- 

 stration that the chromatin substance of a given chromosome forms a 

 unit and that unit persists." Baltzer ('09) was able to recognize in 

 hybrids between Echinus and Strongylocentrotus certain chromo- 



