314 DR. A. MILNES MARSHALL. 



lowing description of the development of the olfactory nerve 

 in the green turtle : — " In embryos of the green turtle of the 

 size of a horse-bean I find the nerves (olfactory) solid. 

 When the embryos are two or three times that size, these 

 nerves each acquire a large cavity proximally, from the fore 

 wall of which the branches seem to spring. The foremost of 

 these branches spring from the top of the vesicle ; they arose 

 at first from the top of the forebrain."^ 



e. General cotisiderations. — Before proceeding to the 

 development of the olfactory organ, I propose to summarise 

 the results to which we have already been led, and to 

 consider briefly certain questions of a more theoretical 

 character. 



The first point I desire to call attention to is the remark- 

 ably close agreement in the mode of development of the 

 olfactory nerves presented by the several types examined, 

 types which, it will be noticed, embrace examples from each 

 of the vertebrate classes, with the exception of Mammalia. 

 In all these types alike — dogfish, trout and salmon, axolotl, 

 frog, lizard, turtle and chick — the mode of development is 

 fundamentally the same ; while the resemblance between the 

 dogfish and the chick, the most generalised and the most 

 specialised of these types is, as I have already shown, 

 complete. I would direct special attention to this agreetnent 

 as affording very strong testimony of the correctness of my 

 observations. 



The fundamental points common to all the above types 

 are the following : — 1, the olfactory nerves appear very early ; 

 2, they are at first connected with the forebrain, and iiot 

 with the cerebral hemispheres ; 3, they are solid, and agree 

 completely in histological characters with the other cranial 

 nerves; 4, an olfactory lobe, when present at all, does not 

 appear till an exceedingly late period of development. 



Though the several types agree so closely in the above 

 fundamental points, they present well-marked differences 

 among themselves. The dogfish appear to form a central 

 type round which the others may be grouped, and from which 

 they may be supposed to be derived. Curiously enough, 

 of the other types the chick appears to resemble the dogfish 

 more closely than any of the others do, with the possible 

 exception of the lizard and turtle, whose earlier stages are as 

 yet unknown. The Amphibia are chiefly characterised by 

 the extreme and long persisting shortness of their olfactory 



' "On the Development of the Skull and its nerves in the Green Turtle 

 {Chelone Midas):' 'Proc. Royal Society,' 1879. 



