NOTICES OF BOOKS. xxv 



The immediately succeeding chapters contains a complete descrip- 

 tion and classification of cell organs into persistent and non-persistent 

 types of the different forms of spheres, and much interesting matter 

 relating to the other constituents of the cell. In chapter viii we find a 

 full description of" cell division and development" and in the succeeding 

 chapters there is a very clear and readable discussion of the several 

 theories of development, through all of which the author's own special 

 knowledge is utilised in full, as in the passages dealing with the nature 

 and causes of differentiation, the Promorphological relations of Cleavage 

 and many more. 



The whole work thus forms a most useful resume of a large subject, 

 the literature of which is so voluminous that it continually threatens to 

 obscure the contained substratum of observations, and an epitome of 

 this sort is a most important acquisition, as it brings the questions which 

 legitimately spring up, when all the observations in each department are 

 taken together, clearly into view. 



Der Liclitsinn augenloser TJiiere. Eine biologische Studie. Dr. 

 Willibald Nagel. Jena: Gustav Fischer, i8g6. 



The author here reprints an interesting and useful lecture on " Sight 

 without Eyes". This lecture, which takes up 48 of the 120 pages of 

 his work, is, as the title signifies, a general account of what is known as 

 the dermatoptic function. The second part gives a detailed account of 

 the author's experiments on Molluscs, which, though blind (naturally 

 or artificially), exemplify this function by reacting in various definite 

 ways to the light stimulus. The third part is made up of appendices 

 which are expansions of important points dealt with too briefly in the 

 lecture. The work closes with a useful bibliography of some eighty 

 references dealing with the physiology of sensation, chiefly of light. 

 These range from Johannes Muller's classical Znr vergleichenden 

 Physiologie des Gesichtssiniies (1826J to the author's own recent 

 contributions. 



From this outline of the contents it will be seen that the work does 

 not pretend to be much more than a review and discussion of facts 

 already known, viz., that certain lower forms of animals without eyes 

 have acquired the power of reacting to sudden changes in the intensity 

 of light. As such, however, it is timely and interesting. Biologists 

 will be grateful to the author for pointing out the interesting fact that 

 those animals which possess hard, protective shells into which they can 

 retreat and defy their enemies mostly react to sudden diminutions of 

 light, e.g., to passing shadows, such shadows no doubt warning them 

 of the approach of a foe. On the other hand, animals which generally 

 live under sand, from which they occasionally emerge, react on sudden 

 brightenings of the light. The author distinguishes between these two 

 diflerent nerve reactions by the use of the words Lichtenipjindiut'j- and 

 Schattenempfindung. 



