290 



SCIENCE. 



[N. S. Vol,. III. No. 60. 



.translation into English has been published by- 

 Prof. Mark, based upon the third German edi- 

 tion. The book has already an assured and 

 high place, and is so well known that it is only 

 necessary to state that its typography and gen- 

 eral appearance have remained unchanged. 



In the new edition many much needed im- 

 provements have been made, and several parts 

 have been entirely recast to concord with the 

 latest progress. The revision has touched es- 

 pecially the following parts: the problem of re- 

 duction-division; the role of the centrosome 

 in impregnation, the development of the middle 

 germ layer in reptiles and mammals, the struc- 

 ture of the chorion, the origin of striated 

 muscles, of blood corpuscles and the develop- 

 ment of the vesicula; and there is one entirely 

 new section, which will be welcome to many 

 embryologists and bears the title, ' Experiments 

 and theories on the significance of the first- 

 formed cleavage cells and of single parts of the 

 ovum for the formation of the organs of the 

 embryo.' There have been so many researches 

 in this field, and they bear so directly on Weis- 

 mann's and other theories of heredity, that the 

 synopsis given by Hertwig will appeal both to 

 those actively at work in this comparatively 

 new region of embryo-mechanics, and to those 

 who wish to learn what conclusions have been 

 reached up to the present. We may also note 

 that since the first edition the number of illus- 

 trations has risen from 304 to 384, and the 

 number of pages from 507 to 612. 



There are certain general criticisms to be 

 made upon Hertwigs' text-book. It is certainly 

 a defect that the author leans far too much 

 upon both diagrammatic pictures and upon 

 diagrammatic explanations, and does not allow 

 free play to actual observation. This is disas- 

 trously the case in his sixth chapter in which he 

 deals with the origin of the middle germ-layer 

 (coelom theory), and by artful shading misrep- 

 resents the actual facts in a manner which is 

 inexcusable even in a text-book. Facts ought 

 never to be mixed with error merely because 

 such dilution serves to hide their discordance 

 with the author's theoretical views. The same 

 tendency to uphold his ccelom theory shows 

 itself in another way in that he still entirely 

 separates the mesenchymal tissues and the 



mesothelial (to which latter he erroneously re- 

 stricts the term mesoblast, p. 115), although it 

 was proven several years ago so as to be past 

 doubt, that Hertwig' s view was unjustified and 

 that mesenchyma and mesothelium are parts of 

 the same layer or mesoderm. This result is not 

 a matter of opinion, it is simply a matter of di- 

 rect observation. This conclusion Hertwig has 

 admitted, yet fails to make it the basis of his 

 exposition, and instead continues the unnatural 

 separation of the two portions of the middle 

 germ layer, much to the confusion of young 

 students. 



Other unfavorable criticisms may be made in 

 regard to special parts of the subject treated 

 inadequately. Such parts are: 1. The nervous 

 system, in that he fails to bring out the funda- 

 mental division into dorsal and ventral zones, 

 or the existence of the three primary layers of 

 the medullary wall, or the significance of the 

 neck-bend, or the history of the neuromeres. 

 2. The fact that the nails are modifications of 

 the stratum lucidum of the epidermis, a very 

 important m.orphological fact. 3. The develop- 

 ment of smooth muscle. 4. The history of 

 the group of connective tissues. 5. The account 

 of the formation of the renal tubules is erroneous, 

 and is the most serious defect noticed by the 

 reviewer. 6. The origin and significance of 

 the yolk cavity and its fusion with that of the 

 notochordal canal in Anura and Amniota to 

 form the definite entodermal canal is not dis- 

 cussed, yet it is a very important point in the 

 morphology of the higher vertebrate embryos. 

 These and other examples which might be 

 given show that Hertwig is far from giving a 

 well-rounded presentation of our present knowl- 

 edge, and that very much needs to be added to 

 make it a thorough and comprehensive treatise. 



In spite of these limitations, Hertwig' s Em- 

 bryology is a text-book of the first class, and 

 has done and will probably long continue to do 

 much for the promotion of the branch of science 

 with which it deals. The treatment of the 

 subject is fresh, origiual, strong and well pro- 

 portioned, so that the leading points receive due 

 emphasis. In many parts Hertwig speaks with 

 the highest authority, notably in regard to the 

 earlier stages of development, and the history 

 of the genital products. The illustrations are 



