212 Transactions of the 



the soft brain substance as solid as cheese without shrinking it ; 

 and that it gives a rich umbre tint to thin laminae of bone, so that 

 in opaque sections the finest layer — the merest trace of a membrane- 

 bone — can be seen, I have tried a solution of chromate of potassae 

 with a httle sulphate of soda added, as recommended in ' Strieker's 

 Histology,' but I have not found any particular advantage in its 

 use above that possessed by the acid. 



Now for the facial arches. The young salmon has one more 

 arch in front of the mouth, and one more behind, than the frog, 

 that is to say, the larval frog : it has, under the head, nine arches 

 in all, two in front of the great mouth-slit, and seven behind it. The 

 first arch, or pair of rods, is the trabecular arch formed by the 

 " rafters of the cranium " ; the second is the pterygopalatine ; 

 the third, the mandibular ; the fourth, the hyoid ; and the remain- 

 ing five are the branchial. The last arch is imperfect and function- 

 less as to respiration. 



Speaking of the science of " form," or morphology, let me say, 

 in passing, that it would be a very simple matter if the primary 

 form were fixed ; but tliis is seldom the case, and the original 

 parts undergo a large series, in many cases, of changes, both in form 

 and tissue. This is the case most remarkably in the facial arches 

 of the osseous fish, especially in the two in front and the two 

 behind the mouth. Yet the primary form of these nine arches is 

 the same, as my simple diagram will show (Plate CII., Fig. C). 



My earliest observations on these have been made upon very 

 young, thin, unsymmetrical embryos, with a rudimentary solid heart, 

 and with the head flat at the top, and just projecting fi-ee from 

 the yolk-membrane. The arches were distinguishable by being 

 granular, but hollow, lying in the midst of, and enclosing, nearly 

 liquid protoplasm. The foremost point most forwards, below, and 

 the hindermost are placed almost transversely across the rudi- 

 mentary throat ; but they all have one shape, viz. that of the 

 letter S, the upper part being most hooked inwards. 



The first pair, the "rafters," together, have a lyre-shaped 

 appearance as they diverge a little in front, and are strongly 

 bowed behind. The next, or palatine pair, are at first merely 

 semicircular, but they become S-shaped afterwards. All those 

 behind the mouth have a remarkable similarity of form, although 

 the two first of these are larger than those that follow ; they all 

 gradually decrease in size, from before, backwards. In the verte- 

 brate animals, generally, these arches have the same form, that is, 

 as far as our researches go ; the amount of modification possible is 

 therefore somethmg marvellous. I was not at aU surprised to find 

 this S-shaped hooked form of arch in the gill-apparatus of the fish, 

 because there it is persistent, and the inturned tops of the bars are 

 bare of gills and carry teeth, which are antagonized by the teeth of 



