TETRAD-FORMATION.
In cases where reduction in the \Veismann sense is actually known to take place, there have been wide variations in the ac counts of the process. It was first described by a former pupil of Weismann's, Ishikawa, who did not find tetrads and who held that reduction in the copepod Diaptomus is accomplished by the separation of entire chromosomes. This result is entirely contra dictory to the more recent results obtained by the subsequent study of Diaptomus and other copepods.
Vom Rath, apparently the first to correctly interpret the for mation of tetrads, gave a different account of reduction in the in sect Giyllotalpa. He found that the spireme is double before it breaks up and that there are half as many of the double segments as there are chromosomes in the somatic cells. The halves of the double segment separate, except at the ends, and a ring is formed. Later the tetrads arise by concentration of the chromatin at four points of the ring, a method by which the four parts of the tetrad originate by longitudinal division represented by the original divi sion of the spireme, and by a transverse division.
Hacker gave still another description of tetrad-formation in Cyclops stremms. His details have been denied by Riickert, who, however, accepted the general results and agreed with him that the tetrad is formed as in Ggilota/pa by a longitudinal and a transverse division of the original spireme segments. Ruckert (1893 and 1894) has found two modes of tetrad formation, each giving, however, the same results, viz.: the formation of tetrads by a primary longitudinal and a secondary transverse division of the spireme. The first method (Cyclops, Cant/wow/pus) agrees almost exactly with what I have here described as the " rod type." The spireme is divided longitudinally before segmentation into half the normal number of chromosomes. The chromatin then begins to collect into a much thicker double rod ; the rod then divides transversely and the tetrad is formed by longitudinal and transverse division. The other method described by Ruckert takes place in Hetcrocqe and Diapteinirs and agrees very closely with what I have described as the " ring type." The double spireme breaks as before, but the double segment, instead of re maining contiguous throughout, becomes separated in the middle, while the ends alone remain in contact. A ring is thus formed and tetrads arise later by two divisions, one through the diameter of original cleavage, the other at right angles to this line.
Enough has been given in this account to show that certain methods tetrad-formation are characteristic of certain species of animals. Up to the present time it has always been found that the tetrads in a single nucleus are formed by one method, either by the " rod type " or by the " ring type " alone. My observa tions on the fern, however, show that tetrads in the same nucleus may be formed by both methods or even by a third. From these facts the conclusion seems inevitable that all the types of tetrad formation mentioned above are merely modifications of the same process and have no significance in themselves so long as a com mon result is obtained.
It seems remarkable that such obvious structures as tetrads should have been hitherto overlooked in the plant reproductive cells. Many observers have noticed that the mitosis in the sporocyte differs from that of all other cells, whether somatic or archesporial. This difference was early recognized by Guignard and Strasburger. Overton and lielajeff also were struck by the peculiar shape and appearance of the chromosomes in this mitosis ; the latter especi ally described them as agreeing in all particulars with Flcmming's account of heterotypical mitosis. Finally Farmer 0895), in speak ing of the pollen-mother-cell of Lilium Hartagon, refers to structures which can be explained only as early stages in tetrad formation : " Die Form der Chromosomen ist sehr unregelmassig, zuweilen erscheinen sie als Bander, oft als Ringe mit eincr oder zwei Protuberanzen, letzteres tritt namentlich in etwas spateren Stadien auf. Ich babe viele Zeit geopfert, mit dem Versuch zu einer festen Entscheidung dariker zu kommen, ob die Ringahnliche Form wirklich primitiv vorhanden oder einer inneren Spaltung zu zuschrciben ist, die das Chromosom noch nicht vollstandig ge theilt hat. Ich neige stark zu letzterer Annahine und betrachte desshald die Ringform, wo sie verkornmt als ein friihes Anzeichen der Langstheilung des Chromosoms " (p. 58). Again he says : Es braucht kaum bemerkt zu werden, dass diese heterotype Form der Mitose auf die Pollenmutterzellentheilungen sich beschrankt und sich weder in den vegetativen, noch in den friiheren Arche sporialen Theilungen derselben Pflanze findet " (p. 64). And finally : " Was die zweite Kerntheilung in Pollenmutterkorn be trifft, so zeigt sie gar nichts von den eigenthiimlichen (hetero typischen) Vorgange, welche die erste Mitose characterisiren, sie weicht nur durch die behaltene reducirte Chromosomenzahl von einer vegetativen oder eine friThen Archesporialen Kerntheilung ab. Es 1st clatter wahrscheinlich, dass die der ersten Theilung besonderen eigenthumlichkeiten mit der plbtzlichen Chromosomen zahlveranderung in einer directen und causalen Beziehung stehen " (p. 67).
The still later work of Miss Sargant ('95), on the chromosome of the pollen-mother-cell of Lilzum shows that a trans verse division of the chromosomes probably takes place. She does not mention the formation of rings which were described by Farmer as preceding the nuclear plate stage, nor does she men tion tetrads. But as Wilson first pointed out ( '96 p. 197), her description of the dividing chromosomes give strong reason to believe that these structures are to be interpreted as tetrads.
The fact that so many observers have described phases which suggest more or less clearly the formation of tetrads in different groups of plants, together with my own observations on the ferns, render it probable that further study will show the reduction of chromosomes through tetrad formation to be a phenomenon of as wide occurrence among plants as it is already known to be among animals.