IV. Comparison of Results.
Dr. R. von Erlanger (’97, 1, and ’97, 2) and Dr. F. Meves (’00) have given excellent discussions of the literature on the nebenkern. I cannot do better than direct the attention of other investigators to them. Hence, I shall discuss only such authors and such points as have special bearing on my own results. The spermatid changes in the Gryllidæ have been studied, so far as I know, only by St. George (’67) and by vom Rath (’92).
St. George, in his early paper, described the transformation process in the “Hausgrille” along with other insects. He found the “Nebenkörper,” and saw that it took a part in forming the envelope of the axial filament, a small particle remaining near the nucleus and other drops of it appearing at intervals on the tail. In his paper on Blatta (’86, 1) he accepted Bütchli’s name, “nebenkern,” and traced its origin to the spindle remains. In his paper on Phratora (’86, 2) he takes up the description of the nebenkern in the spermatocytes and spermatogonia. In the spermatid, he describes the blackberry stage in these words: “Während das andere (not nucleus) Körperchen weniger lebhaft gefärbt als ein kleiner Fadenknäuel mit unregelmässigem Contour erscheint.”
There can be no true nebenkern in the spermatogonia and spermatocytes, as St. George first used the word, and as I have limited its application. But St. George found the object, traced its origin and fate, and described several of the stages as they appear in Gryllus.
Vom Rath (’92) found Gryllotalpa very poor material for following the spermatid changes. However, he described an oval body which he found varied in appearance, depending on the method of fixation. He calls it the “nebenkern” and thinks it goes to help form the tail.
The description is too incomplete for comparison. Judging from Gryllus, I question its being very poor material.
Bütschli (’71) studied the spermatid transformations in Acrididæ and Locustidæ along with that of other animals. He saw the object formerly described by St. George and named it “nebenkern.” He saw it divide into halves, elongate, and form the spermatozoon tail.
The dividing of the nebenkern into halves seems to be an appearance quite common in insect spermatogenesis. I have myself seen it in several genera of Acrididæ—Hippiscus, Arphia, Melanoplus, and Brachystola; besides Bütschli, St. George, Henneguy, Platner, Paulmier and others have described it.
Platner in his studies has given special attention to the nebenkern. In his first paper on “Pulmonates” (’85) he did not trace the origin of the nebenkern, but described it as consisting of four to six rods of different lengths and irregularly bent. These were connected, forming an irregular polygon. In its later stages he saw it with a mass of protoplasm pass down along the primary tail—an early protrusion of protoplasm. Finally it is lost.
In his succeeding paper (’86, 1) he studied the “nebenkern” spindle remains—in the spermatogonia and spermatocytes of pulmonates. In the spermatids, as in the former generations of cells, the nebenkern grows out of the nucleus, where it, with the chromatin, had formed the spireme. It appears as a loop, which becomes larger, twisted, and entangled, and finally breaks loose from the nucleus. Later it goes to form the spiral covering of the primary tail, changing it to the axial filament and true tail.
In his next paper (’86, 2) he describes the changes when the dividing of the protoplasm lags behind in the spermatocyte divisions. His description agrees in so many points with my own, that I shall quote his exact words:
“Die Spindelfasern hingegen contrahiren sich mehr und mehr nach dem Equator hin, wobei sie mit ein ander verschmelzen und merkwürdiger Weise je weiter dieser Verdichtungsprocess fort schreitet um so mehr an Tinctions fähigkeit speciell gegenüber dem Hämatoxylin gewinnen. Sie stellen jetzt zwei dreieckige oder hakenförmige Gebilde dar, die mit der Spitze noch im Equator zusammenhängen mit der breiten Seite sind sie den zugehörigen Zellen zugewendet. Hier sind ihre Grenzen undeutlicher, verwaschen und zeigen hier auch noch häufig eine streifige Beschaffenheit, welche auf ihren Ursprung hin weist. Zuweilen lassen sich einzelne Fäden noch eine beträchtliche Strecke weit in das Protoplasma hinein verfolgen, welches zwischen ihnen und den sich ausbildenden Zellkernen liegt.”
The “häkenformige Gebilde” moves away from the periphery, its sides elongate, break, and unite at the nuclear end, thus form the polygonal nebenkern. “Derselbe geht also in diesem Falle direckt aus den Spindelfasern hervor.” When the protoplasm does divide a similar process takes place. The spindle remains divide at an equatorial line and each half forms a nebenkern, and, as he says: “Also auch hier geht der Nebenkern direckt aus den Spindelfasern hervor. Vielleicht geht in den Spermatiden der Nebenkern überhaupt immer aus den Spindelfasern hervor, in dem die langfädige Verbindüngsbrücke, die ihn oft mit dem Kern verbindet, sich wohl als ein noch einige zeit persisttirender Rest der esteren deuten lässt.”
In his paper on Limax (’89, 1) he followed the nebenkern through all the divisions and thinks it a constant organ of the cell. In the second part of the paper on Helix and Paludina the nebenkern was considered as formed from the remains of the spindlepole and the centrosome. Later Platner (’89, 2) found the nebenkern in the pancreas cells. In reports upon Pygæra and Sphinx he changed the name nebenkern to mitosome. The centrosome lies in front of the nucleus and forms the point of the head. This he calls the nebenkern. From the spindle remains arise two bodies. A large, fibrous one from the equatorial end has a clear space around it and the axial filament passes through it. It soon elongates and forms the tail. This is the large mitosome. The other is much smaller and arises from the polar end of the spindle fibers. It takes its place in the angle between the large mitosome and the nucleus. Here it persists till the nucleus begins to elongate, when it lengthens and surrounds the basal end of the spermatozoon tail. This is the small mitosome.
Platner (’85) saw a true nebenkern. I have already criticized the use of the word “nebenkern,” as the name for the spindle remains in the spermatocytes. Platner himself later denied the nuclear origin of the nebenkern. His results (’86, 2) agree with mine concerning the uniting, converging, staining and bending of the spindle-fiber remains and their passing to the tail. Again, Platner (’89, 1) probably followed the centrosome and attraction sphere, as well as the nebenkern, in the second part of the paper on Helix and Paludina. In the pancreas the body is a result of secretion, and is not a nebenkern. In Pygæra Platner’s large mitosome is the real nebenkern, as shown by its fibrous structure, its surrounding clear space and destiny. His small mitosome is what I have described as the acrosome, and he is mistaken as to its final use. The body he called centrosome in the nucleus is the persisting accessory chromosome. Such would be my interpretation of his figures.
Henking (’91) has followed the spermatid changes in Pyrrhocoris quite carefully. He finds that the fibers between the chromatin masses are separated into peripheral fibers and central spindle fibers. The first, a part of the second and the yolk mass, forms the nebenkern, which passes down over the axial filament. The rest of the central fibers form the mitosome. This takes its position at the angle between the nebenkern and the nucleus. On the surface of the nucleus it passes to the anterior end, then back to its original position. A piece now is constricted off and disappears, while the rest, increasing its affinity for stains, again wanders to the anterior pole of the nucleus, and becomes the acrosome.
The large amount of yolk substance is a disturbing element in Pyrrhocoris, but the nebenkern agrees with that of Gryllus in having the same origin and destiny, as does also the acrosome in fate and position. Besides, there is much similarity in the stages, as seen by comparing Henking’s fig. 63 with my [fig. 25], and his fig. 85 with my [fig. 32].
Wilcox (’95 and ’96) described the spermatid metamorphosis in Caloptenus. The interzonal fibers, a long, striated body composed very plainly of distinct fibers, contracts longitudinally, and the corners round themselves off, and this forms the nebenkern. It loses its fibrous structure, comes close to the nucleus, and then grows out into the axial filament. The centrosome moves half way around the nucleus and lies between the latter and the nebenkern. Later the centrosome becomes the middle piece.
This description does not give much detail nor do the drawings show the stages clearly. As far as given, the formation of the nebenkern is the same as in Gryllus; but in Gryllus the axial filament is not formed from the nebenkern, as can be plainly seen from [fig. 17].
Erlanger (’96), in a short paper, discusses the use of the term “nebenkern,” and suggests limiting it as is done in this paper. He opposes St. George’s opinion, that the nebenkern comes from the cytomicrosomes. In Blatta the cytomicrosomes are preserved during the whole process of mitosis, and have no connection with the spindle fibers, but during the telophase they collect in reduced numbers around the daughter nuclei.
In 1897 he called the collection of granules around the centrosome, the centrodeutoplasm. He considers them to be identical with St. George’s cytomicrosomes and the archoplasm (or attraction sphere) of other writers. In order to harmonize results, he suggests that, since the centrosome sometimes wanders around the nucleus, the centrodeutoplasm (or sphere) may unite with the spindle remains in some cases to form one body, as shown by the descriptions of Henking, Henneguy, Meves, and others. He later (’97, 2) describes the so-called “sphere,” and distinguishes between it and the true nebenkern.
I would strongly commend his excellent discussion of literature and his careful comparison of the results of investigators. He has shown clearly that the nebenkern comes from the spindle remains.
Calkins (’96) finds that the nebenkern comes from the spindle fibers and is useless in the cell. But Lumbricus is peculiar in having the nebenkern simply disintegrate, for, in many cases, he admits that the nebenkern has an important function. I do not have access to Henneguy’s or Bolles-Lee’s or Toyama’s works, yet I should judge from Erlanger’s and Meves’s criticism that all of these have the nebenkern originate from the spindle remains, and Henneguy describes it as having a “fibrillar appearance,” and Bolles-Lee as “fibrillar structure.” Accordingly, I think that each of these has discovered the correct origin of the structure, and I do not doubt that there is, at least in the first two, a more or less direct change from the spindle remains to the nebenkern.
Paulmier (’99) finds that in Anasa the nebenkern comes from the yolk mass and remains of the spindle fibers. A part of this mass separates off, while the whole is still in a confused condition, and forms the acrosome. The nebenkern forms the tail sheath, while the acrosome forms the point to the head.
A comparison of Paulmier’s fig. 42 with my [fig. 4] suggests the thought that they are the same stage, and his fibers are remains of the spindle. His fig. 43, of course, agrees with my [fig. 25]; and fig. 43 may correspond to [figs. 26 or 14], only that his stain is weaker.
Meves’s investigations are the most extensive of the recent ones on the nebenkern. He has used Paludina and Pygæra as his objects. The consideration of the mitochondrion in the spermatocytes I shall pass over, as it is not within the bounds of this paper; yet I hope to study the earlier generations of my material, and shall then compare the results. In Meves’s description of Paludina, I find but one point in which it agrees with Gryllus. The head of the young spermatozoon in Paludina has a clear space in the center filled with nuclear fluid, which remains till the head begins to stretch. In Gryllus it is hollow until maturity.
In Paludina the nebenkern in one kind of spermatids is formed from threads made up of mitochondria—small, round bodies identical with St. George’s cytomicrosomes and Erlanger’s centrodeutoplasm. These threads change to vesicles, which, reduced to four, closely surround the centrosome as it lengthens into the middle piece. At first they form a four-cleft cylinder, but finally a single sheath. An idiozome and spindle remains are seen in the spermatid. They persist for awhile, and the former changes into the acrosome. In Pygæra Meves finds two kinds of spermatids, distinguished by a small difference in size. The larger forms the typical spermatozoon. The spindle remains form a “Spindelrest körper,” which is soon lost. The ends of the mitochondrion chains form a ring of dark mass surrounding a clear space. The ring is broken by radiating clear spaces. These spaces collect and unite into larger vacuoles, which surround the darker center. Finally there is only a dark ball with a surrounding clear space shut in by a ring. The centrosome with attached axial filament fastens itself to the nucleus; then both begin to grow longer. The axial filament passes over the surface of the ring surrounding the ball—the “mitochondrion körper” or nebenkern. This body elongates a great deal then the darker mass puts forth numerous threads which surround the axial filament.
As to Pygæra, the peculiarity is the complete agreement of his “mitochondrion körper” and my nebenkern in appearance and behavior for a part of the transformation and their complete disagreement in the other part. The question with me is, “Do the two bodies whose final stages are so similar originate so differently, or has one of us mistaken the origin of the body?”
Since reading Meves’s paper I have carefully reexamined my material, and I am positive that I am right as to the origin of the nebenkern; but, on the other hand, I would not say that Meves is wrong in his observations, as in doing so I should fall into the same error which I think Meves himself has made. In his discussion of the literature he has forced every description to agree with his ideas, or has declared that the author has described some extra-nuclear organ as a nebenkern which is not a nebenkern. Thus by implication, if not by direct statement, he says that a nebenkern never comes from the spindle remains. With due respect to his ability and long experience as an investigator, I must say that Meves is mistaken in this. St. George himself traced the cytomicrosomes back to the spindle remains. Besides, the many investigators whom I have cited above cannot be mistaken as to the origin of the nebenkern. In my own material I am positive that there is a direct passing of the interzonal fibers over into the earlier stages of the nebenkern.
That the body which forms the tail covering does not come from the spindle remains in all cases, I am willing to admit. Meves has cited many investigators, especially on vertebrates, whose results favor such an opinion. The spindle remains do not change into a nebenkern even in all Arthropods, as Blackman (’01) finds no nebenkern, nor anything in anywise resembling it, in Scolopendra.
From my study of the results of other investigators, it is evident to me that there are at least two general methods for the formation of the covering for the spermatozoon tail. One of these plans will harmonize Meves’s mitochondrion körper, Erlanger’s centrodeutoplasm, Heidenhain’s pseudo-chromosomes, and other similar structures. The other will show that Platner’s large mitosome, Paulmier’s blackberry stage and my striated nebenkern are only different stages of the spindle remains changing into the tail covering.