174 Papers from the Marine Biological Laboratory at Tortugas. 



careful search, not the slightest evidence of such an element in its usual 

 morphological form and peculiar behavior with reference to spireme and 

 idiosome could at any stage be detected ; but this negative evidence obviously 

 can not remove all uncertainty with respect to this first point. 



Heterochromosomes (allosomes, Montgomery) in mongoose, and other 

 forms showing like growth-stage conditions, may possibly have disappeared 

 by close union with one of the ordinary chromosomes and thus forced into 

 the usual behavior of the autosomes during auxocyte phases. Transition 

 stages in such process of disappearance are apparently shown in certain 

 orthoptera (McClung, 1905) and amphibia {e. g., Necturus, King, 1912). 



Militating further against the objection of mistaken identity is the fact 

 that in this group the plasmosome has almost invariably disappeared^ before 

 synapsis, when the X-element first exhibits its characteristic behavior. If 

 plasmosome could be confused with X-element in mammals, a group of five, 

 with no relatively closer affinity than obtains among the contrasted group, 

 could hardly be expected to give consistent and unequivocal evidence in 

 favor of complete absence. Moreover, if the plasmosome generally disap- 

 pears before or about the time of synapsis in a relatively no more closely 

 related group of five forms, it seems quite legitimate to infer that it disap- 

 pears at about the same time in six other — and as closely related — forms, 

 and thus leaves little opportunity for confusion. And the above deduction 

 is supported to some extent by direct observations (exception: man). 



Having disposed of these objections as far as possible within the limita- 

 tions of microchemical technic, it remains to consider further bearings and 

 implications of the facts established. The observation of an X-element in 

 the ovary of young cats (fig. 42) by Winiwarter and Sainmont (1909) has 

 important significance in this connection. If my negative evidence for the 

 male germ-cells in the cat can be regarded as equally certain with the 

 positive evidence of these investigators for the female germ-cells, then it 

 would seem that in those forms in which the males lack a typical X-element 

 this is possessed by the female.^ An analogous instance from the inverte- 

 brate group is that of certain sea-urchins reported by Baltzer (1909). 

 Here the female possesses a pair of idiochromosomes, while the male lacks 

 any indication of such. The above hypothesis for mammals must be tested 

 by examination of the young ovaries of those forms in which the male 

 appears to lack the accessory. 



This hypothesis would be somewhat weakened if cases were known in 

 which both male and female germ-cells possessed typical' X-elements. 

 The meager evidence available is contrary to such a hypothetical condition. 

 Wilson (1905, 1906) was unable to find a chromosome nucleolus in certain 

 genera among the insects, including Euchistus, in which he was confirmed 

 by Foote and Strobell (1909). Buchner's (1909) evidence with respect to 

 Gryllus — the only presumptive contradictory evidence known to me — 



* A delayed disappearance occasionally occurs, more especially in cat and man. 



* Presumably by approximately half of the ova, 



' That is from the standpoint of behavior during the growth stages. 



