“Le ‘chromosome accessoire,’ découvert par McClung chez Xiphidium fasciatum, se retrouve chez les locustiens que nous avons étudiés. Chez Orphania, il se divise dans les spermatogonies en deux masses volumineuses et allongés, que l’on reconnait dans les nucléoles, également volumineux et allongés, des spermatocytes de premier ordre en prophase. A la métaphase de la première cinèse, on le trouve situé excentriquement et plus près de l’un des pôles; il va tout entier a l’une des cellules-filles. Dans celle-ci, il se divise comme un chromosome ordinaire, d’où il suit que sur quatre spermatides formant la descendance d’un spermatocyte, deux se trouvent privilegiees. Par ce partage inégal, non réalise dans Xiphidium fasciatum, d’après McClung, le chromosome spécial d’Orphania rappelle celui des hémiptères.”
A like series of processes is recognized in the Phasmids.
As is elsewhere explained in this paper, the occurrence of two divisions of the accessory chromosome in Xiphidium, which was mentioned as a possible occurrence in my preliminary paper, is shown not to take place. While it is much more difficult to demonstrate the undivided condition of the accessory chromosome in one of the spermatocyte mitoses of Xiphidium than it is in the cells of Orchesticus, Anabrus, Scudderia, and Microcentrum, I am convinced that it does not differ from the other Locustids in this respect.
We may therefore feel assured that our knowledge of the morphological character of the accessory chromosome in the Orthoptera is fairly well established. This gives us a good base from which to conduct further comparative studies into other groups, and it is to be hoped that our knowledge of this element will rapidly increase.
Unfortunately, de Sinéty has chosen to add another name to the already overburdened list of synonyms, and “chromosome spécial” now takes its place in the literature of insect spermatogenesis. The reason for adding this name—
“Il reçu successivement leg noms de ‘accessory chromosome’ (McClung), ‘small chromosome’ (Paulmier), ‘chromatin nucleolus’ (?), ‘chromosome x’ (Montgomery). Nous avons préféré éviter ces appellations, qui semblent toutes supposer une signification qui n’a jamais été définie ou s’appuyer sur des caractères plus ou moins secondaires, pour adopter un nom indifférent, celui de ‘chromosome spécial,’ nous conformant à l’idée de Wilson, pour qui c’est un ‘extra chromosome,’”
would seem to be at least insufficient, since “accessory chromosome” can scarcely be regarded as implying any more primary or secondary function than can “chromosome spécial.”
(f) Individuality of the Chromosomes.
In each of my preceding papers I took the opportunity to point out the fact that, even were the accessory chromosome of no other value, it would certainly be worthy of study for the light it throws upon the question of the individuality of the chromosomes. On this point Montgomery has much to say in his late paper (15). I think it cannot be questioned that we have here indisputable proof that at least one chromosome may be identified through all the cell generations of the testis. While this does not prove that chromosomes are persisting and independent structures, it does evidence the fact that they may be, and greatly strengthens the hypothesis that they are.
In addition to the evidence here offered by the accessory chromosome, there must be noted that derived from a study of spermatocytes in which there is always present one ordinary chromosome that greatly exceeds the others in size. Such a condition is found in the cells of Anabrus. The disproportion in size of the elements is here so striking that it would be impossible to fail in distinguishing the giant chromosome. In each of the spermatocytes of Anabrus there are therefore two chromosomes which are plainly recognizable. It may be observed further that the remaining chromosomes are quite different in size, and it may be possible within reasonable limits of certainty to pick out one or more other chromosomes in each cell. Unless this could be done for each element, however, it would not definitely prove that all the chromosomes are distinct and recognizable structures. The actual recognition of two elements in each cell of the same generation and its ancestors or descendants in other generations goes far, however, to render probable the individuality of each chromosome.