ZOOLOGY AND BOTANY, MIOROSCOPY, ETC. 
607 
chain of cells which runs forward from it, similar to the chains produced 
by the teloblasts of Lumbricus or Clepsine. By this growth the teloblasts 
are pushed back over the mesoderm cells, and the mesoderm is forced 
below the surface of the egg. There is no invagination of the blastodisc 
whatever, but the ectoderm simply grows back over it. In the case of 
the endoderm cells there is actual immigration, the cells sinking down 
into the interior of the yolk and becoming the “ vitellophags.” 
The cells of the mesoderm plug, soon after their overgrowth by the 
ectoderm, scatter, and the majority pass forwards to form the mesoderm 
of the head and anterior body region ; others, which apparently take on a 
teloblastic function, are carried back along with the ectodermal teloblasts, 
and give rise to the mesoderm of the posterior part of the body. 
Though the statements of the author are, in many respects, very 
divergent from those of previous writers on the embryology of Isopods, 
they may, he thinks, serve to bring into closer harmony the modes of 
formation of the germ-layers of those Crustacea, of which we possess an 
adequate account. 
Spermatogenesis, Oogenesis, and Fertilization in Diaptomus. * — 
Prof. C. Ishikawa has made a study of the development of the repro- 
ductive elements in a Japanese species of Diaptomus. He finds that the 
primary sperm-cells correspond exactly with the primary egg-cells ; each 
contains eight chromatic elements. In both sets of cells the eight 
chromosomes become constricted transversely so as to form eight dumb- 
bell-shaped bodies. After division has occurred two or three times, 
the resulting cells form the mother-cells of eggs or of spermatozoa. 
These, after growing considerably, begin to divide as before. In the egg 
this stage corresponds to the formation of the first polar body. Without 
any intervening resting stage of the nuclei there immediately follows 
another division, and there is a reduction in the original number of chro- 
mosomes (Weissmann’s “ Beduktionstheilung ”). During the formation of 
the second polar body the sperm-cell enters the egg-cell. The nucleus of 
the sperm-cell is, ordinarily, at first rather small, and stains deeply and 
homogeneously, but differentiation soon sets in, and the four chromo- 
somes become distinctly visible. The female pronucleus shows at first 
distinctly four elements, which soon grow longer, become much convo- 
luted, and pass into a “ skein ” stage. After the formation of the 
equatorial plate the two nuclei unite ; the number of chromatic elements 
in each of the copulating elements is now found to be eight, and in the 
first two segmentation spheres there are likewise eight chromosomes. 
Nuclear Division in Cyclops. f — Dr. Y. Hacker has studied the 
processes of nuclear division in the formation of the mesoderm and 
endoderm in Cyclops. 
In Cyclops brevicornis , before the beginning of gastrulation, a single 
cell, appearing in the blastocoel, divides into a central and a peripheral 
portion by normal mitosis. The central cell (A) divides again, in 
heterotypic fashion, and forms the genital cells. From the peripheral 
portion (B) arise the two primary mesoderm cells. In the division of 
the cell ancestral to A and B the fine coil stage becomes a spirem, the 
* Journal Coll. Sci. Imp. Univ. Japan, v. (1892) pp. 1-34 (1 pi.) 
f Arch. f. Mikr. Anat., xxxix. (1892) pp. 550-81 (2 pis.). 
