If, therefore, Montgomery’s recent observations are correct, it must follow, I think, that his “chromatin nucleolus” and the accessory chromosome are different structures. I am free to admit, however, that his statements are far from convincing. So much dependence is placed upon the numerical relationships of elements that are admittedly very minute, and so little corroborative proof is given, that I entertain serious doubts as to the accuracy of the observations. In this connection I would suggest a comparison between the figures of the “chromatin nucleolus” in the first paper upon Euchistus (figs. 55–68) (12) and those in the last one (figs. 1–17) (15). The showing here made would alone be sufficient to raise a question as to the nature of the “chromatin nucleolus,” and until further evidence is forthcoming the character of the peculiarly modified chromosomes in the spermatocyte of the Hemiptera must remain in doubt.
Aside from definite retractions that Montgomery has made regarding his earlier views on the character of the “chromatin nucleolus,” there are noticeable different attitudes toward it in his earlier and later works. Thus, in his lecture at Woods Holl (13a), we find the following: “These remarkable ‘nucleolar’ structures which stain like chromatin have been observed by numerous writers, but as yet no satisfactory description has been given of their mode of origin. They have been observed by me in spermatocytes of various insects, in hypodermal and other cells of Carpocopsa, and in follicle cells of the testicles of Plethodon and Mus.” At this early stage of Montgomery’s investigations it is apparent that he views his “chromatin nucleolus” primarily as a nucleolus with chromatic origin and characters, but the fact is equally apparent that he now regards it primarily as a “chromosome” with nucleolar attributes. This is made evident in his recent definition, which reads: “The chromatin nucleoli are morphologically chromosomes, undergoing division in mitosis like the other chromosomes, but differing from them in the rest stage by preserving a definite (usually rounded) form.”
What has here been said regarding the “chromatin nucleolus” applies to those structures in Euchistus and other Hemiptera to which Montgomery has given the name without qualification. According to his definition, however, there is present in the cells of Protenor and other species another form, the “chromosome x.” Not only by inference is this classification operative, but by direct statement we learn that Montgomery regards this element as a member of the class of bodies which he calls “chromatin nucleoli.” In speaking of Protenor chromosomes, he says: “This is the only case in the Hemiptera where one chromosome becomes differentiated into a ‘chromatin nucleolus’ for the first time in the spermatocyte generation.”
The noteworthy thing about this “chromosome x” is the fact that in every essential detail it corresponds to the accessory chromosome of the Orthoptera. It is a spermatogonial chromosome that comes over intact into the spermatocyte; it retains its form and staining power unchanged through the prophase of the spermatocyte; it divides in only one of the spermatocyte mitoses; and is a large and conspicuous element of the cell at all times.
This “chromosome x” agrees just as closely in its description to the accessory chromosome as do the ordinary ones of the two orders, and, if Montgomery’s account is correct, there would seem to be no reason for doubting their identity. In two respects, however, there are differences between these structures. First, it is to be noted that the “chromosome x” divides in the first spermatocyte, while the accessory chromosome undergoes separation in the second spermatocyte. Should Montgomery’s observations prove correct, it would yet indicate no fundamental difference in the character of the element, for the result is the same whether division takes place in the first or second mitosis. In either event, one-half the spermatozoa are provided with the odd chromosome while the remaining half are not.
The second point of difference would seem to be a more serious one. Montgomery states that during the spermatogonial mitosis the “chromosome x” regularly divides as do all the other chromosomes, i. e., longitudinally. In the spermatocyte mitosis, however, the element is broken across, and the longitudinal split, which is apparent in the early stages, disappears and is not utilized in division. We have here the remarkable occurrence of a chromosome entirely unchanged in its structure, but merely differing in its surroundings, which, instead of dividing along the plane marked out for it, as it has done in all preceding mitoses, breaks across after it is a formed element. An occurrence of this kind, so different from the usual method of division, would require strong proof to establish it, and this, in my opinion, Montgomery has not brought forward.
A criticism of the degeneration theory as advocated by Paulmier and Montgomery has already been given (17), so that it would not be necessary to consider it here except in so far as it has been modified since its promulgation. As a rule, Montgomery refers to his “chromatin nucleoli” throughout his late paper (15) as degenerating chromosomes, but in discussing their function specifically he makes important changes in this conception. These are stated as follows: “When we find, accordingly, the mutual apposition of them (true nucleoli) to chromatin nucleoli, it would be permissible to conclude that the chromatin nucleoli are chromosomes which are especially concerned with nucleolar metabolism. And this, I think, would be the correct interpretation. The chromatin nucleoli are in that sense degenerate that they no longer behave like the other chromosomes in the rest stages, but they would be specialized for a metabolic function; and from this point of view they would certainly seem to be much more than degenerate organs.”
It is difficult to comment upon a contradictory statement like this; but, fortunately, it is not necessary to do so, since it carries with it its own refutation. The conception of a chromosome specialized in the direction of increased metabolic activity as being in the process of disappearing from the species can hardly be regarded seriously.
Taking everything into consideration, it may be said that Montgomery’s work upon the Hemiptera has left the subject in a very disturbed condition, and any prospect of a complete agreement between the accessory chromosome of the Orthoptera and the “chromatin nucleolus” of the Hemiptera is made more remote than was previously the case. This, I think, is largely due to the inferior character of the Hemipteran material, which has lead to misconception of phenomena that are clearly marked in Orthopteran cells.
It is gratifying to note that the recent work of de Sinéty (37) practically corroborates the conclusions herein set forth regarding the history of the accessory chromosome. Aside from failure to observe the important spireme condition of this element in the first spermatocyte prophase, de Sinéty describes the same series of processes with scarcely an exception. His summary contains the following account of the accessory chromosome: