spermatogenesis of the Mongoose, etc. 173 



Regarding a possible confusion between plasmosomes and accessory 

 chromosomes, or X-elements, it must be noted first that in the stain em- 

 ployed (Heidenhain's iron-hematoxylin ; checked in doubtful instances by 

 Auerbach's methyl-green-acid-fuchsin stain) the plasmosome when present 

 (as in spermatogonia and occasionally in primary spermatocytes) is always 

 in properly decolorized preparations considerably less deeply stained than 

 the X-element. Moreover, the latter is frequently attached to the spireme 

 or one of its segments, and generally holds a position close to the nuclear 

 wall and close to the point where the idiosome lies. Again, it is frequently 

 bilobed or compound. In certain instances, e. g., mouse more particularly, 

 one of the first-division chromosomes occasionally passes to the pole in 

 advance of the mass; but in no instance could the X-element be followed 

 with any satisfaction or certainty beyond the early prophase. At this stage, 

 however, it is in at least a number of instances clearly recognizable among 

 the pale, mossy chromosomes as the body earlier identified as the X-element. 

 The evidence would seem amply adequate to support the interpretation 

 here as a typical X-element. 



Heterotropic nuclear elements must be identified by other criteria than 

 staining reaction. At best, staining capacity can only give confirmatory 

 evidence. Morphological marks would seem to be the most certain grounds 

 for basing distinctions. The complete nuclear history is of course necessary 

 for full certainty. Chromosomes undergo alterations in their chromaticity 

 and consequent staining capacity at different phases of the nuclear cycle. 

 Again, true chromatin-nucleoli occur in certain forms (e. g., oocytes of 

 Asterias forbesii, Jordan, 1908; oocytes of Echinaster and Cribrella, Jordan, 

 1 910), whose function at least in part is to contribute chromatic material 

 to the chromosomes just before they enter the first maturation spindle. 

 The plasmosomes of certain forms may thus be of true chromatic nature. 

 Moreover, true nucleoli never assume — except perhaps very exceptionally 

 and atypically — the characteristic bilobed and split forms of the chromo- 

 some-nucleoli during the growth stages. Degenerating nucleoli become 

 ragged of outline, frequently fragment, and undergo karyolysis, but never 

 show the series of phenomena characteristic of heterochromosomes : sharp 

 contour, deep basophily, frequently split form, frequent attachment to 

 spireme, and usual location close to the nuclear membrane. A presumptive 

 chromosome-nucleolus may meet the test of basophilic staining reaction in a 

 differential dye, but if it does not meet at least the majority of the above 

 structural and spatial tests it very probably is simply a chromatic plasmo- 

 some; on the other hand, if it answer to the latter tests, confirmatory — but 

 perhaps not crucial, in view of the undoubted chemical changes which 

 chromosomes undergo — evidence accrues from a chromatic reaction to a 

 "specific" stain. 



Regarding the first stated point of possible doubt, then, nothing further 

 can be said than that in the five forms enumerated as lacking an X-element, 

 the tissue was similarly well preserved and stained, but notwithstanding 



