Memoirs: The Spermatogenesis of Lepidosiren Paradoxa

Abstract

The different generations of cells composing the germinal epithelium, resemble those often described in other forms, especially in the Amphibia. Very little arrangement of the different generations in different parts of the testis could be observed. The somatic number of chromosomes is thirty-eight. One pair of these is conspicuously larger than the rest. The reduced number of chromosomes in the bouquet stage appears to be arrived at by a parallel conjugation in the early prophase according to von Winiwarter's scheme. In strepsinema, which synchronises with the onset of synizesis, the conjugants separate except at their ends, to form very long-drawn-out rings. During synizesis and diakinesis the rings break into their constituents, and the somatic number of univalent chromosomes is again obtained, the "homologous" chromosomes being often widely separated from each other. During diakinesis each univalent becomes divided by a transverse constriction, which probably corresponds with the apices of the V's of the pre-meiotic chromosomes, and also with the transverse division of the Copepod type of tetrad, which cannot therefore be taken as indicating the point of junction of two chromosomes united end to end, As in Copepods, the transverse constriction is not the division plane in either mitosis, but disappears during anaphase II. After the dissolution of the nuclear membrane "homologous" chromosomes are seen to approach each other, and join together a second time to form the rings or modifications of them found in metaphase I. The first maturation division separates entire "homologous" chromosomes. There is no resting stage between the two divisions. In the second division the chromosomes divide longitudinally, forming "tetrads," etc., very like those of metaphase I. A resting or semi-resting stage may be intercalated into the spermatocyte diakinesis. In this condition the chromosomes, in the somatic number, are distributed round the periphery of the nucleus just under the nuclear membrane. In advanced stages they lose their regular shapes and become connected by numerous anastomoses. In one testis several spermatocytes in this stage have taken on all the characters of oocytes. The spermatogonial prophases are of a very simple nature, and show no sign of anything comparable to the stage of zygonerna. In the spermatogonial and also in the somatic nuclei the chromosomes are arranged in a definite plan, the smaller and larger ones being grouped together. Within the main groups there is also evidence of a tendency for chromosomes of equal size to be next to each other.