Eyes of Molluscs and Arthropods. * 733 



Tecliuique. 



Notwitlistauding- tlie fact tbiit a cousidevable number of workers 

 bave given tbeiv attention to tbe study of tbe eyes of Perfen aiid related 

 genera, no progress in our knowledge of tbese iiiteresting organs bas 

 beeu made sinee Hensen publisbed bis studies upon tbe eyes of tbe 

 Cepbalopods; indeed many points, clearly demonstrated by bim, were 

 by later writers entirely overlooked. 



If we bave made any addition to tbe knowledge of tbis subject. it 

 is only because tbe treatment to wbicb the tissues are now subjeeted 

 allows US to demonstrate witb ease wbat , twenty years ago , vvould 

 bave been impossible. As it required mucb time and patienee to lind 

 a reagent witb wbicb tbe eyes could be properly preserved, it will, 

 perbaps, not be amiss if I sbould describe tbe metbods by wbicb tbe 

 resiilts given above bave been obtained. 



In tbe study of tbe various components of tbese complicated 

 Organs, it was necessary to resort to different metbods for tbe study of 

 dififerent parts; no one treatment was found by wbicb all tbe facts 

 given could be observed. Almost any of tbe ordinary reagents would 

 preserve sufficiently well tbe cells of tbe cornea, iris and lens. More 

 care in tbe cboice of fixing reagents was necessary for a proper treat- 

 ment of tbe outer ganglionic layer and tbe retinal cells, wbile it was 

 only after repeated trials of all tbe reagents at my command, witb 

 tbeir various combinations and metbods of application . tbat I was, at 

 last, enabled to obtain tbe rods, witb tbeir complex network of gangli- 

 onic fibres, in a tolerable state of preservation. 



Tbe eyes of Pecten bave been studied fresb , as well as by means 

 of sections and maceration. The principle underlying- my whole metbod 

 of treatment, wben sections have not been used, bas been to harden 

 tbe tissues a very little, and then macerate. After stainiug in 

 picro-earmine, wbicb I consider in such casestobe better than alcobolic 

 Solutions, tbe isolated cells are examined in a medium withavery 

 low index of refraction; eitber distilled water or tbe macerating 

 fluid. Wben I desired to keep tbe preparations for furtber study, 

 I was obliged to use acetate of potash, a good preserving- medium witb 

 a low index of refraction, but for soft macerated cells, it must be used 

 witb great care, for it often causes considerable shriukage. 



Tbe young Pectens, from 1 to 3 mm long, were thrown into a 

 mixture of equal parts of concentrated sublimate and picro-sulpburic 



