750 



SCIENCE. 



[N. S. Vol. XX. No. 518. 



qualitative, in the second case quantitative 

 only, thus giving the immediate possibility 

 of the production of two embryos from a 

 single egg. It appears to me that we find 

 here a principle of reconciliation between 

 the hypothesis of mosaic development and 

 pre-localization, and the apparently contra- 

 dictory one of non-mosaic or correlative 

 difEerentiation. The facts show that each 

 of these apparently contradictory hj'poth- 

 eses contains an element of truth ; that we 

 must recognize in the development of every 

 animal the fact of pre-localization and of 

 mosaic development, but also the fact of 

 correlative action. The relation between 

 these two can not be predicted, but must be 

 determined in each individual case; for 

 the knov/n facts are already sufficient to 

 prove that the segregation of the forma- 

 tive stuffs is a process that occurs at dif- 

 ferent periods in different animals. At 

 the time of fertilization, accordingly, the 

 segregation differs both in degree and in 

 form; and these differences have not yet 

 been reduced to any general law. 



In conclusion, I would express the opin- 

 ion that, so far as the early stages of de- 

 velopment are concerned, it is difficult to 

 escape the hypothesis of formative stuffs 

 or specific morphoplasmic substances, in 

 some form. But while this hypothesis 

 facilitates an understanding of the modus 

 operandi or immediate causes of differen- 

 tiation, it leaves us as much as ever in the 

 dark as to the localizing or form-deter- 

 mining factors which are responsible for 

 the determination of the segregation pat- 

 tern. This problem, which is essentially 

 one of correlative action, is not only un- 

 solved, but suggests the existence of specific 

 energies for which it is difficult at present 

 to find an analogy outside the field of pi'oto- 

 plasmic action. 



Edmund B. Wilson. 



colambia uwiveesity. 



SCIENTIFIC BOOKS. 

 Contributions to the Study of the Behavior 

 of Lower Organisms. By Heebekt S. 

 Jennings. Published by the Carnegie In- 

 stitution of Washington. 1904. 256 pis., 

 81 text-figures. 



In the series of papers which make up this 

 volume Professor Jennings presents results 

 which are of fundamental importance for our 

 understanding of the behavior of lower organ- 

 isms. The titles of the seven papers of the 

 volume are as follows : (1) ' Reactions to Heat 

 and Cold in the Ciliate Infusoria,' (2) ' Ee- 

 actions to Light in Ciliates and Flagellates,' 



(3) ' Reactions to Stimuli in Certain Rotifera,' 



(4) ' The Theory of Tropisms,' (5) ' Physiolog- 

 ical States as Determining Factors in the Be- 

 havior of Lower Organisms,' (6) ' The Move- 

 ments and Reactions of Amosba,' (7) ' The 

 Method of Trial and Error in the Behavior of 

 Lower Organisms.' 



For the purposes of this review the papers 

 may be separated into three groups. Of 

 these the first, which includes the first three 

 papers, is devoted primarily to descriptions 

 of the modes of reaction of several of the 

 lower organisms, and to a discussion of the 

 bearing of these reactions upon the ' orien- 

 tation theories ' of Loeb and Verworn. The 

 second group is constituted by the paper on 

 Amcfiba; in it the author deals in detail, as a 

 result of his own observation, with the me- 

 chanics of locomotion, modes of reaction and 

 psycho-physiology of the organism. Papers 

 four, five and seven are included in the third 

 group. In addition to presenting several 

 points of interpretation, they contain dis- 

 cussions of the relations of the author's results 

 to the general theory of tropisms. 



I shall now attempt to state briefly the prin- 

 cipal points made in each of these three sub- 

 ject divisions of the volume. 



In explanation of the directive influence of 

 stimuli on the movements of various organ- 

 isms Loeb, Verworn and others have proposed 

 the so-called orientation theories. 



According to these theories a stimulus which 

 acts unequally upon different portions of the 

 body causes inequality of contraction in the 

 musculature, and thus brings about a turning 



