128 



THE POPULAR SCIENCE MONTHLY. 



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 per- 

 haps 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 exceeding- 

 ly difficult. The number of visible stars is 

 estimated by the author as seventy millions. 

 Outside of this finite universe there may ex- 

 ist 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 nebu- 

 lae accompany the text ; an index and notes 

 are also added. 



Human Embryology. By Charles Sedg- 

 wick Mikot. Illustrated. New York: 

 William Wood & Co. 1892. Pp. xxiii 

 + 815. 



The appearance of another work on em- 

 bryology justifies the assertion that was re- 

 cently 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 announce- 

 ment 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 in- 

 vestigations and to collecting and reviewing 

 the literature of the subject, is presented in 

 this splendid volume that is a worthy repre- 

 sentation of American scholarship and re- 

 search. 



On account of the intimate relations be- 

 tween the uterus and the embryo, the author 

 devotes his first chapter to a careful presen- 

 tation 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 re- 

 trogressive and progressive histories of the 

 foetus and its envelopes. 



The author calls attention to the limita- 

 tion of the term genoblast to the sexual ele- 

 ments proper, to the spermatozoon or the 



egg-cell after maturation, and not to the sper- 

 matophore or the egg-cell before maturation. 

 The subjects of spermatozoa, ova, ovulation, 

 and impregnation are described with refer- 

 ence to the latest investigations. The author 

 believes that the ovum draws the spermato- 

 zoa toward itself by chemical influence, act- 

 ing 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 so- 

 lution of the origin of sexuality the attractive 

 hypothesis is offered that sexuality is coexten- 

 sive 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 evi- 

 dence 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 forma- 

 tion of the ccelom and mesothelium, and the 

 origin of the mesenchyma, are carefully de- 

 scribed in connection with a review of the 

 principal theories in regard to the morpho- 

 logical significance of the mesoderm, the au- 

 thor believing that Hatsehek's germ-band 

 theory offers the best-founded explanation 

 of the vertebrate mesoderm. 



Emphasis is laid on the fact that the 

 splanchnocoele (pleuroperitoneal cavity) is al- 

 most, if not quite, from the start divided into 

 a precociously enlarged cervical portion (am- 

 nio-cardial vesicles), and a rump portion (ab- 

 dominal cavity), the boundary between the 

 two portions being marked by the omphalo- 

 mesaraic veins, that run from the area vas- 

 culosa 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 pro- 

 liferation of the endothelial lining of the ves- 

 sels, 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 



