Calcareous Parts of Lantern of Aristotle in Echinus miliaris. 469 



satisfactorily by ordinary methods ; and even if they could be sectioned, there 

 was the enormous difficulty of reconstructing, either mentally or otherwise, a 

 very complicated system of ossicles. But, happily, these difficulties were 

 obviated by following a unique idea very kindly suggested to me by Prof. 

 MacBride. He thought that if one could cut away just the top part of the shell, 

 then, after some slight maceration, one could see the lantern ossicles below. 

 By employing this method were obtained several important facts which are 

 recorded in this paper. I here take the opportunity of expressing my deep 

 indebtedness to Prof. MacBride for the many valuable suggestions xnade and 

 for the frequent advice given throughout the course of this investigation, and 

 for kindly reading the manuscript. 



2. Method. 



The process of double imbedding in celloidin* and paraffin was employed, 

 and the " Jung " sliding microtome was used for sectioning. The latter 

 instrument exactly suited my purpose, viz., making an opening at the aboral 

 surface of the young urchin. Any slight error in orientation can readily be 

 corrected in the course of sectioning by the side-to-side and front-to-rear 

 adjustments of the object-holder. I must remark, however, that it required 

 some skill and judgment to cut away just enough of the aboral part of the 

 shell so as nob to damage the lantern below. 



When enough of the shell had been sectioned off, the paraffin was dissolved 

 out in xylol, and the celloidin was removed by using a mixture of equal parts 

 of ether and absolute alcohol. The young urchin was then passed through 

 decreasing strengths of alcohol and finally brought into distilled water when 

 it was ready to be treated with " Eau-de-la-barraque." In this liquid was 

 discovered an ideal macerating solution for the young urchin material. 

 While it does not act so vigorously as caustic-potash, for instance, it dissolves 

 in a given time just enough of the tissues to enable one to see through them 

 the contained transparent ossicles. This maceration was watched under a 

 low power of the microscope and it lasted never more than a few seconds. 

 The young urchin was quickly transferred to distilled water and passed 

 through increasing strengths of alcohol, then into xylol, and finally mounted 

 in xylol-balsam in the cavity of an excavated slide. Such whole mounts, 

 when illuminated with a " Nernst " electric lamp and examined with a 2 mm. 

 oil immersion lens, show the ossicles of the lantern very well indeed. 



Entire embryonic teeth were obtained by applying gentle pressure, with a 

 mounted-needle under microscopical guidance, to a well-macerated young 



* The imbedding in celloidin was done according to Gilson's rapid process (Lee's 

 ' Vade Mecum,' 8th edition, p. 104). 



