No. I.] OPTIC LODE LAYERS IN LOWER VERTEBRATES. 3 



There is in the Tropidonotus cortex a striking change in the 

 appearance of the first gray layer, at the point midway between 

 the inner and outer edges of the cortex. Outward from this 

 point, the cells are considerably larger than those inside ; they 

 take a deeper carmine stain, and their cell-processes are very 

 much more distinct. The change is rather abrupt. These cells 

 continue downwards to the ventral outer corner of the hemi- 

 sphere, and at this point terminate close to the surface. 



Much the same structure obtains in Eviys {Fig 3), excepting 

 the last-mentioned characteristic, viz., the change in the size 

 and appearance of the cells of the first cortical layer. There 

 is also an indefinite fibre-layer between the first and the second 

 cortical layers (/"). 



3. AvES. — In Columba livia (Fig. 4) there are four layers 

 corresponding approximately to those of the reptiles, but the 

 cells composing the gray layers are very much more numerous 

 than in reptiles ; and, in fact, than in any other forms I have 

 studied, not excepting DidelpJiys. First, there is a thin super- 

 ficial fibre-layer ; then comes the first gray layer, occupying not 

 less than three-fifths of the entire thickness of the mantel. 

 Some of the cells in this layer present a triangular appearance, 

 but as far as I could ascertain, they are not of the pyramidal 

 variety found in the mammalian cortex. The second gray 

 layer is composed of cells running in the same direction as the 

 fibres of the corpus callosum. Most of them are lenticular in 

 shape, but there are a number that are less elongated. 



4. Mammals. — A lower type of the class — the opossum — 

 was selected in order the more easily to find the homology, 

 should any exist, to the preceding forms. In the opossum there 

 are seven layers altogether : I, superficial fibre-layer ; II to V, 

 four gray layers ; VI, fibre-layer of the corona radiata ; VII, the 

 ependyma (Fig. V). 



The first gray layer (or the layer II of the figure) consists of 

 numerous pyramidal cells among which are also seen some of the 

 ordinary spherical cells. The pyramidal cells have each a nucle- 

 olus, and their apices are turned toward the surface. The layer 

 III is composed exclusively of ordinary nerve-cells, and their 

 processes present no definite trend. In this layer vertical fibres 

 are visible, as is also the case with the layer II. The layer IV 

 resembles the II in its composition ; only the pyramidal cells 



