498 



SCIENCE. 



[N. S. Vol. XXIII. No. 587. 



The building up of the corallum is followed 

 out in detail through the formation of the 

 third cycle of permanent septa. Among the 

 illustrations of this part of the work special 

 mention is due the microphotographs of 

 macerated skeletons of developing polyps, and 

 the figures of living polyps with the beginning 

 skeleton in situ. Much interest attaches to 

 Professor Duerden's account of the develop- 

 ment of the septa. It has been hitherto as- 

 sumed that the septa of a new cycle appear in' 

 the exocQ3les (i. e., the space between two 

 pairs of mesenteries), but are later embraced 

 by the newly appearing pairs of mesenteries 

 in such wise as to lie in the entocoales (i. e., 

 the space between the mesenteries of a pair). 

 Thus the same septa would be first exocoelic 

 and then entocoelic. In opposition to this 

 scheme Duerden's observations lead him to 

 the conclusion that while exosepta are formed 

 in successive cycles, they never become ento- 

 septa. The cycles of entosepta are strictly 

 new formations, appearing as do the primary 

 six septa in entocoelic spaces. The succession 

 of the cycles of exocoelic septa is maintained 

 through the continued peripheral bifurcation 

 of preexisting exocoelic septa. The bifurcated 

 extremities become the (exoccelic) septa of a 

 new cycle, while the main septum is incor- 

 porated in the growing body of one of the last 

 formed cycle of entosepta. Having respect 

 only to the actual facts as observed in 8ider- 

 astrea, it has been found that any one of the 

 permanent septa, later than the first six, has 

 a double origin. It is in part a new forma- 

 tion (entocoelic), and in part a preexisting 

 formation (exocoelic). The two parts fuse, 

 and the fusion is interpreted by Professor 

 Duerden as the incorporation by a growing 

 organ of the remnant of a vanishing organ. 

 In a developing corallum according to this 

 view exosepta are formed at each stage of 

 growth, only to disappear as the permanent 

 septa, entosepta, come into existence. Thus 

 the development of coral septa affords an ex- 

 cellent example of substitution : temporary 

 organs precede and are replaced by perma- 

 nent organs performing the same function 

 as the former. As a corollary to this conclu- 

 sion the author expresses his belief that the 



exoseptal predecessors of the permanent septa 

 do not wholly disappear in all corals, as inde- 

 pendent structures, but persist in some species 

 in the shape of the pali found in front of the 

 larger septa. H. V. W. 



Die Erystallgestalten der Mineralogie in 

 StereosTcopischen Bildern. Von Professor 

 Theodor Haetwig, Professor at the Staats- 

 realschule in Steyr, Upper Austria. Vienna, 

 Verlag von A. Pichler's Witwe und Sohn. 

 As the author says in his description of the 

 set,^ the method of stereoscopic illustration 

 has been applied within a few years in medi- 

 cine, in technical science, as for instance in 

 drawings of machines, for the representation 

 of microscopic objects, and in the measure- 

 ment of terrestrial and astronomical distances. 

 In this application to crystallography Pro- 

 fessor Hartwig has prepared 120 stereographic 

 drawings of crystals, which are printed on 

 white cardboard and placed in a simple stereo- 

 scope with adjustable focus, the whole packed 

 in a neat box. 



The drawings are divided into two sets, 

 {A) the simple crystal forms and separate 

 projections of the axes of the sis systems, and 

 {B) the more usual combinations in examples 

 of natural crystals. The usual clinographic 

 projection is used, modified of course for the 

 stereoscope, and in all cases the axes are pro- 

 jected inside the forms. The effect of solidity 

 produced by these drawings and the clearness 

 of the relations shown is surprising; in some 

 ways they surpass the usual glass models with 

 colored threads inserted to represent the axes. 

 In the development of hemihedral forms from 

 the corresponding holohedral the effect is 

 particularly good and yet something is left 

 to the imagination of the student in com- 

 pleting their derivation. The drawings are 

 intended to supplement solid models, espe- 

 cially in individual instruction, and perhaps 

 in some cases to replace the more expensive 

 glass models, as with the hemihedral forms 

 mentioned above. 



John E. Wolff. 



' Zeiischrift fiir Lehrmittehoesen und padagog- 

 ische LUeratur, Vol. 1, No. 7, 1905, pp. 217-220. 



