Page:Popular Science Monthly Volume 43.djvu/139

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and the peripheral portion the vascular wall, and in birds the red cells arise from the walls of the venous capillaries of the bony marrow. In other words, the blood-cell is a liberated, specialized endothelial cell.

One of the most interesting and valuable chapters in the volume is that on the germinal area and the embryo and its appendages, in which there is a synopsis of the published descriptions of embryos not over three weeks old; from these it is learned that no human ovum has been observed to have a primitive streak, which is the first stage of the series formulated by the author. In this stage (twelfth or thirteenth day) the human ovum is a rounded, somewhat flattened sac of three or four millimetres in diameter, bearing an equatorial zone of short, unbranched villi that are probably formed by the ectoderm only; the wall of the sac is ectoderm, whether underlaid by somatic mesoderm or not is uncertain; a mass of cells is attached to the inner wall of the sac, over one of the bare poles of the ovum, constituting the rudiment of the embryo. The second stage is characterized by the appearance of the medullary plate, the third by the appearance of the medullary groove, the fourth by the formation of the heart and medullary canal, the fifth by the development of the first external gill-cleft, the sixth by the appearance of two external gill-clefts, the seventh by the appearance of three gill-clefts, and the eighth by the appearance of four external gill-clefts.

The fourth part of the work includes descriptions of the chorion, the amnion and proamnion, the yolk-sac, allantois, and umbilical cord, and the placenta.

The final portion of the volume is devoted to chapters on the growth and development of the various organic systems of the fœtus.

Each section and chapter aims to present a comprehensive review of the literature regarding the subject therein considered, the author stating the reasons for accepting certain theories in preference to others. One blemish in the volume is the free use of German embryological terms. The author's devotion to German has often led him to use, also, forms of expression that, while correct in German, are faulty English. This is, however, a minor and remediable fault in what is a most excellent book.

Pioneers of Science. By Oliver Lodge, F. R. S. London: Macmillan & Co., 1893 Pp. 404. Price, $2.50.

This work consists of a course of eighteen lectures on the history and progress of astronomical research, with biographical sketches of each pioneer and an examination of their influence on the progress of thought. It is divided into two parts. The first, which is entitled From Dusk to Daylight, contains ten lectures giving a brief outline of the physical science of the ancients, with an interesting account of the progress of astronomy from Thales, 640 b. c., to the death of Newton, 1727 a. d. The second part is called A Couple of Centuries' Progress, and embraces the period of astronomical discovery from the publication of Newton's Principia to the present time.

The author shows considerable power of lucid condensation in his description of the labors of the early astronomical scientists, and while giving a brief history of their discoveries—notably those of Archimedes, Ptolemy, and Roger Bacon—he brings us at a bound over the void of the middle ages to the beginning of the sixteenth century (1543) when Copernicus (Nicolas Copernik) published his famous work, De Revolutionibus Orbium Cœlestium, in which he proved that the earth is a planet like the others, and that it revolves round the sun—thus shattering the accepted Ptolemaic system and revolutionizing all other (speculative and theological) doctrines concerning the form of the earth and the motion of the heavenly bodies.

This period is called by Mr. Lodge "the real dawn of modern science." His sketch of Tycho Brahé is most interestingly written; and in the summaries of facts which preface each lecture will be found some curious coincidences of the dates of the birth and death of the famous philosophers from Copernicus to Newton. While admitting the great labors and immense value to astronomical research of Galileo's discoveries, the author does not class him with Copernicus, Kepler, or Newton; in fact, he says that "Archimedes and Galileo can only be considered in the light of experimental philosophers." Lord Bacon, who flourished about the same time as Descartes, is very summarily dismissed; he does not admit him into his list of philosophers, and says: "His (Bacon's) methods are not