Page:Popular Science Monthly Volume 43.djvu/138

128 the magnesium flutings are not accepted by other astronomers, and experiments do not confirm his explanation of the aurora. Most puzzling of all astronomical problems perhaps is the arrangement of stars. If we could observe from some other point in the heavens the system might be disclosed to us, or even if we could compute the distance of every star, the design might appear. In all cases, however, the parallaxes are so small that the measurements are exceedingly difficult. The number of visible stars is estimated by the author as seventy millions. Outside of this finite universe there may exist vast systems in space whose light has not yet reached us, or which may be forever hidden, because light itself is extinguished in a separating void.

Some fine photographs of stars and nebulæ accompany the text; an index and notes are also added.

appearance of another work on embryology justifies the assertion that was recently made in these columns that there was a growing appreciation of the importance of this subject. The present volume has been expected for some time past, as the announcement was made some years ago that Prof. Minot was engaged in the preparation of a work upon this topic. The ten years' labor that has been directed to making original investigations and to collecting and reviewing the literature of the subject, is presented in this splendid volume that is a worthy representation of American scholarship and research.

On account of the intimate relations between the uterus and the embryo, the author devotes his first chapter to a careful presentation of the anatomy and the histology of the uterus, together with a description of the changes that occur during pregnancy. In the second chapter there is a general outline of human development, in which there are retrogressive and progressive histories of the fœtus and its envelopes.

The author calls attention to the limitation of the term genoblast to the sexual elements proper, to the spermatozoön or the egg-cell after maturation, and not to the spermatophore or the egg-cell before maturation. The subjects of spermatozoa, ova, ovulation, and impregnation are described with reference to the latest investigations. The author believes that the ovum draws the spermatozoa toward itself by chemical influence, acting as an attracting stimulus, in a similar manner to the attraction Pfeffer has shown certain chemical substances may have for moving spores; the attractive power of the ovum being annulled or weakened by the formation of the male pronucleus. As a solution of the origin of sexuality the attractive hypothesis is offered that sexuality is coextensive with life; that in protozoa the male and female are united in each of the conjugating cells, and impregnation is double; and, finally, that in the metazoa the male and female of the cells separate to form, genoblasts or true sexual elements, and impregnation is single.

The author presents a great deal of evidence to support the theory that concrescence is the typical means of forming the primitive streak in the vertebrate, the primitive axis of which is formed by the growing together in the axial line of the future embryo of the two halves of the ectental line.

The origin of the mesoderm, the formation of the cœlom and mesothelium, and the origin of the mesenchyma, are carefully described in connection with a review of the principal theories in regard to the morphological significance of the mesoderm, the author believing that Hatschek's germ-band theory offers the best-founded explanation of the vertebrate mesoderm.

Emphasis is laid on the fact that the splanchnocœle (pleuroperitoneal cavity) is almost, if not quite, from the start divided into a precociously enlarged cervical portion (amnio-cardial vesicles), and a rump portion (abdominal cavity), the boundary between the two portions being marked by the omphalomesaraic veins, that run from the area vasculosa into the embryo proper at nearly right angles to the embryonic axis.

The author agrees with Ziegler that the red blood-cells of all vertebrates arise by proliferation of the endothelial lining of the vessels, basing this conclusion upon the facts that in various vertebrates certain parts of the vascular system are at first solid cords of cells, the central portion becoming blood-cells