SKELETON AND LARYNX OE XENOPUS AND PIPA. 93 



A short trial was given of that method of investigation which 

 is now iinding such favour with embryologists, and which has 

 been employed with such excellent results by Gaupp (14) in his 

 investigations on the hyobranchial skeleton of Banafusca. The 

 head or other part is cut into microscope-sections of known 

 thickness, and the organs to be studied are reconstructed there- 

 from in wax plates, the thickness of which is the same multiple 

 o£ the original as the linear magnification. The wax sheets are 

 then pressed together, and an enlarged model of the organs 

 is thus obtained. "Without in the least wishing to depreciate 

 the results of Gaupp's investigations, for which I have the 

 greatest admiration,! would point out that at the critical periods 

 when the cartilages are forming or are becoming absorbed, their 

 outline is extremely difficult to make out in sections, and that in 

 fashioning the wax plates it is very largely a matter of personal 

 opinion to decide where the line shall be drawn between the 

 cartilage and the surrounding tissues ; and thus an error of a 

 centimetre or more, according to the magnification, may very 

 easily creep into the model. If the sections are cut thin, this 

 difficulty of discrimination increases ; while if the sections are 

 thick, it is almost impossible to represent truthfully in the 

 model narrow rod-like structures which make a small angle with 

 the planes of section. The method is even more tedious than 

 that of actual dissection by reflected light, and far less satis- 

 factory and conclusive. 



I had already completed this portion of the investigation by 

 the time that Wilder's paper (43) reached me; and so I did not 

 include a trial of the method he adopted in his recent work on 

 the adult laryngeal skeleton of Amphibia, viz., that of mounting in 

 turpentine and Canada balsam after slow staining with a weak 

 alcoholic solution of methyl-blue, washing, and dehydrating. I 

 have since made a few experiments in this direction, and can testify 

 to the differentiating action which the dye has upon hyaline car- 

 tilage. The method, however, is useless for the demonstration of 

 cartilage which is just forming or is undergoing absorption, and 

 could not therefore be applied successfully to such purposes as 

 the present. 



S^enopus Icevis. 



Of the large number of tadpoles of Xenopios examined, it must 

 suffice to choose only three stages for minute description and 

 delineation, because in the earlier stages of development the 



