202 Transactions of the 



been copied in a reduced form and tinted by Mr. Geo. West, for 

 ilhistration of this remark. But objects hardened by spirit cost 

 much trouble when they have to be used as sohd specimens, and 

 examined by reflected hght ; the tissues are not half bard enough, 

 and they have lost all colour ; so that it is no easy work to make a 

 dissection from the outside agree, in the mind, with the various 

 sections that have been made of other specimens at the same stage. 

 Some tadpoles hardened in dilute chromic acid were given me by 

 Professor oMichael Foster two years ago ; these opened my eyes to 

 the immense advantage of this hardening material. The objects 

 thus procured are very brittle, but with care the most exquisite solid 

 preparations can be made by tearing away the skin, and teasing out 

 the dry, stick-like muscular fibres from amongst the gelatinous tissue 

 which abounds in the tadpole between the skin and flesh. Then 

 the colour is so rich ; the deep amber tint makes such objects come 

 out in reflected light in a way that leaves the mind quite at ease as 

 to what the eye beholds. Such solid dissections can be made of the 

 skulls of embryos altogether no larger than a mustard-seed, and 

 solid sections made both vertically and transversely come out with 

 exquisite distinctness, corroborating quite what had been determined 

 from the outside. An embryo thus hardened can be held between 

 the finger and the thumb, and then the whole creature can be 

 cloven through, " from the nave to the chops," with a sharp razor. 

 Such vertical sections are easily made by " an old hand," and so are 

 the transverse slices, to make which I begin at the nose and cut 

 away rapidly as large a number as possible from the same specimen ; 

 these are soon valued under the inch lens, and the good ones pre- 

 served in glycerine between glass, the thick glass receiving a j)en- 

 and-ink mark at the time, which saves time \\hilst investigation is 

 going on. Some of these sections are thin enough for transmitted 

 light ; others of almost equal value show well by reflected light. 



I do not examine the sections and dissections which take the 

 reflected hght in glycerine : I preserve them in it, but always wash 

 it away and put them into water, which allows the reflected rays to 

 pass through much more easily, and gives therefore a much clearer 

 view of the object. I nearly forgot to mention that one most de- 

 lightful efi'ect of the chromic acid is to give not only the tissues 

 generally an amber colour, but also the abominable, impenetrable 

 " pigmentum nigrum." Often, in olden times, with spirit prepara- 

 tions, have I been almost exhausted with attempts to decipher parts 

 imbedded in this hateful pigment. 



A figure in my " Shoulder-girdle " paper, which shows the two 

 halves of the embryo frog's sternum,* cost a week's labour, and 

 but for chromic acid I should have given up Frog-morphology in 

 disgust: — how often have I wished that these little savages — 



* See platf 5, fig. 4, of tlint work. 



