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].