172 Papers from the Marine Biological Laboratory at Tortugas. 



DISCUSSION. 



Heterochromosomes are apparently lacking in mongoose, cat, squirrel, 

 pig, and rabbit, and appear to be present in white mouse, sheep, horse, 

 mule, bull, and dog. On the basis of what was already known regard- 

 ing man (Guyer, 1910; Gutherz, 1912),^ rat (Guyer, 1910), armadillo 

 (Newman and Patterson, 1910), guinea-pig (Stevens, 1912), and opossum 

 and bat (Jordan, 191 1: 1912), heterochromosomes were suspected in mam- 

 mals generally. With respect to heterochromosomes, then, mammals fall 

 into two groups: those lacking and those possessing the X-element in the 

 male germ-cells. Under the first group five^ mammals can be classified; 

 under the second, twelve. Both groups include higher as well as lower 

 mammals. Measured by no other criterion known to me can these mammals 

 be similarly grouped. Presence of heterochromosomes obviously bears no 

 relationship to natural affinities or evolutionary levels of mammals. The 

 proportion of 12 to 5 among examined and recorded cases of mammals in 

 favor of those possessing heterochromosomes suggests that in those forms 

 apparently lacking them they have simply eluded detection. They might 

 presumably be so small as to escape recognition, or lack the peculiar 

 morphological marks by which they are usually characterized, or be too 

 labile to be identified. 



Regarding a similar condition in Culex, Stevens (191 1, 1912) suggests 

 that here, "where no heterochromosome differentiation of any kind has 

 been detected, the members of a pair of chromosomes may differ by a sex 

 unit or some other character unit, and the difference in size comes within 

 the probable error of most careful observation" (p. 165). 



The observation of the attachment of the accessory chromosome to the 

 parasynaptic spireme (figs. 28, 29, and 40) controverts the plausibility of 

 Stevens's (1912) suggestion that " the condensed condition of the odd chro- 

 mosome and the unequally paired heterochromosomes of the growth stage of 

 the first spermatocytes might be due to their unpaired or unequally paired 

 condition preventing them from joining with the other bivalent chromo- 

 somes to form a spireme, especially in cases of parasynapsis " (p. 166). 



It would seem to be required, then, for a proper grounding of later 

 hypotheses, that all uncertainty be disposed of, first with respect to the 

 possible presence in some form or at some stage of an X-element in those 

 animals in which it seems to be wanting; and second, with respect to possible 

 confusion with a persisting plasmosome in those instances where an X- 

 element is believed to obtain. Though the above order would seem to be 

 the more logical, the best approach to the argument and the proof is per- 

 haps by way of the second point, though in fact they sustain a reciprocal 

 relationship. 



1 Gutherz (1912) confirms Guyer's (1910) observation respecting the presence of a heterotropic element in 

 man, but interprets it as a pair of equal idiochromosoraes undergoing neither heterolcinesis nor giving rise to 

 a dimorphism of spermatozoa as described by Guyer. 



' This must be a tentative grouping; only four mammals can now be temporarily placed in this group. 



