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.