Literary Notices. vii 



most recent is that of Studnicka^ who has devoted himself especially 

 to filling the gaps in our knowledge of the optic nerve in the Ichthy- 

 opsida and Reptilia. 



Even the most aberrant forms can clearly be derived from the 

 embryonic tubular optic stalk. In Petromyzon the lumen of the stalk 

 soon disappears, but its cells, or their descendants, persist throughout 

 life as an axial core of glia cells in the nerve. In Protopterus there 

 is also an axial strand, but the cells are few and scattered and 

 the nerve is more or less completely separated into strands by connect- 

 ive tissue septa entering from the periphery. In Ceratodus and Lep- 

 idosiren this separation has been carried further until we have very 

 numerous distinct strands each with an axial core of cells. The simp- 

 lest condition is found in Necturus, where the lumen persists through- 

 out life. In other Amphibia there is either an axial mass of glia cells 

 or many such series scattered through the cross section of the nerve, 

 and this is the condition in the higher animals in general. 



In the fishes, however, the deviations from the original condition 

 are very great. Among the selachians, Chimaera has a simple cylin- 

 drical nerve with the glia cells scattered uniformly through it and with 

 no connective tissue septa. In the sharks, however, such septa may 

 be very highly developed, even to the division of the nerve into very 

 numerous small strands with septa between them. This destroys the 

 earlier generalization of DeyF that, as we pass from low to high in 

 the vertebrate series the amount of connective tissue in the opticus 

 progressively increases. 



Polypterus resembles the Selachii more closely than the other 

 ganoids. Acipenser and Polyodon show the band-form which is so 

 characteristic of teleost';, while Lepidosteus carries this type to its ex- 

 treme, the band being very wide and plaited several times. The lat- 

 ter form is very common among teleosts. Studnicka differs from 

 Deyl in regarding it as derived by the folding of a ribbon-shaped 

 nerve, rather than as derived from a cylindrical nerve by the intrus- 

 ion of connective tissue septa. In Hippocampus the whole nerve is 

 broken up into a number of separate strands, but these are shown by 

 their arrangement to be derived from a plicated band by the interpo- 

 sition of complete septa. Deyl attempts to deduce certain phylogen- 

 etic considerations from his study of the optic nerves, but Studnicka 



1 Untersuchungen iiber den Bau des Sehnerven der Wirbeltiere. Jenaische 

 Zeits., XXXI. 



' Zur vergleichenden Anatomic des Sehnerven. Prag, 1895. 



