DIVERGENT ACCOUNTS OF REDUCTION We can only touch on a few of the accounts of reduction which differ from both the modes already considered. Of these the most interesting are observations which indicate the possibility of, The Formation of Tetrads by Conjugation A considerable number of observers have maintained that reduction may be effected by the union or conjugation of chromosomes that were previously separate. This view agrees in principle with that of Ruckert, Hacker, and vom Rath ; for the bivalent chromosomes assumed by these authors may be conceived as two conjugated chromosomes. It seems to be confirmed by the observations of Born and Fick on amphibia and those of Ruckert on selachians (Pristiurns); for in all these cases the number of chromatin-masses at the time the first polar body is formed is but half the number observed in younger stages of the germinal vesicle. In Pristiurus there are at first thirty-six double segments in the germinal vesicle. At a later period these give rise to a close spireme, which then becomes more open, and is found to form a double thread segmented into eighteen double segments ; i.e. the reduced number. In this case, therefore, the preliminary pseudo-reduction is almost certainly effected by the union of the original thirty-six double chromosomes, two by two. The most specific accounts of such a mode of origin have, however, been given by Calkins (earthworm) and Wilcox (grasshopper). The latter author asserts ('95) that in Caloptenus the spireme of the first spermatocyte first segments into the normal number (twelve) of dumbbell-shaped segments, which then become associated in pairs to form six tetrads. Each of these dumb-bell-shaped bodies is assumed to be a bivalent chromosome, and the tetrad-formation is therefore interpreted as follows :— abcd -1 (spireme), ab-cd-kl (segmented spireme), ab/cd, ef/gh, etc (tetrads) There is, therefore, no longitudinal splitting of the chromosomes. A careful examination of the figures does not convince me of the correctness of this conclusion, which is, moreover, inconsistent with itself on Wilcox's own interpretation. Since each germ-nucleus receives six chromosomes, the somatic number must be 12, and Wilcox has observed this number in the divisions of the spermatogonia. The 12 dumb-bell-shaped primary segments must therefore represent single chromosomes, not bivalent ones, as Wilcox assumes, and his primary tetrad must therefore be not ab/cd, as he assumes , but either – a/b or (if we assume that the normal number of chromosomes undergoes a preliminary doubling) aa/bb. Until this contradiction is cleared up Wilcox's results must be received with considerable scepticism.
The second case, which is perhaps better founded, is that of the earthworm (Lumbricus terrestris), as described by Calkins ('95, 2), whose work was done under my own direction. Calkins finds, in accordance with all other spermatologists save Wilcox, that the spireme-thread splits longitudinally and then divides transversely into 32 double segments. These then unite, two by two, to form
i6 tetrads. The 32 primary double segments therefore represent chromosomes of the normal number that have split longitudinally, etc., and ie. a/a - b/b the formula for a tetrad is ab/ab or ax/ax. Such a tetrad, therefore, agrees as to its composition with the formulas of Hacker, vom Rath, and Ruckert, and agrees in mode of origin with the process described by Ruckert in the eggs of Pristiurus. While these observations are not absolutely conclusive, they nevertheless rest on strong evidence, and they do not stand in actual contradiction of what is known in the copepods and vertebrates. The possibility of such a mode of origin in other forms must, I think, be held open.
Under the same category must be placed Korschelt's unique results in the egg-reduction of the annelid Ophryotrocha ('95), which are very difficult to reconcile with anything known in other forms. The typical somatic number of chromosomes is here four. The same number of chromosomes appear in the germinal vesicle (Fig. 96, D). They are at first single, then double by a longitudinal split, but afterwards single again by a reunion of the halves. The four chromosomes group themselves in a single tetrad, two passing into the first polar-body, while two remain in the egg, but meanwhile each of them again splits into two. Of the four chromosomes thus left in the egg two are passed out into the second polar body, while the two remaining in the egg give rise to the germ-nucleus. From this it follows that the formation of the first polar body is a reducing division (!) —a result which agrees with the earlier conclusions of Henking on Pyrrochoris, but differs entirely from those of Ruckert, Hacker, and vom Rath. The meaning of this remarkable result cannot here be discussed. A clue to its interpretation is perhaps given by Hacker's interesting observations on the two modes of maturation in Canthocamptus, for which the reader is referred to Hacker's paper ('95, I).
Moore ('95) has given an account of reduction in the spermatogenesis of mammals and elasmobranchs which differs widely in many respects from those of all other observers. In both cases there is said to be a resting stage between the two spermatocyte-divisions, and in mammals (rat) the reduced number of chromosomes first appears in the prophase of the last division. In elasmobranchs both spermatocyte-divisions are of the heterotypical form, with ringshaped chromosomes. On all these points Moore's account contradicts those of all other investigators of reduction in the animals, and 'he is further in contradiction with Ruckert on the number of chromosomes. His general interpretation accords with that of Brauer and Strasburger, reducing divisions being totally denied. The evidence on which this interpretation rests will be found in his original papers.