268 Oyi the EmhryoJogy of the Bctina of Teleosteans. 



the same time the segments of the cones become visible. 

 We have now six retinal layers, distinguishable by the 

 following characters : — 



1. Co7ie-layer. — Vertically placed, elongated, deeply 

 staining cells, with very slight, fine, clear projections from 

 their outer extremities. Their bases stain most deeply. 



2. External molecular layer. — No longer clear, but now 

 forming a distinct, thin, dark, finely granular line. 



3. Inner granules. — Cells about six deep. 



4. Internal molecular layer. — Uniform, rather thick, non- 

 staining. 



5. Ganglion cell-layer. — Cells about four deep and rather 

 larger than the inner granules. Distinct one-celled row ex- 

 ternally. 



6. Nerve-jibre layer. — A thin, non-staining streak, with 

 horizontal fibrillation just evident. 



The pigment occurs in distinct, flat, horizontal, long-oval 

 deposits. These nearly all touch one another, so as to form 

 an almost continuous layer traceable forwards to the iris. 



Outside the pigment layers of fibrous-looking tissue are 

 now formed, which can be traced forwards to the front of the 

 equator of the lens, where it is anterior to the pigment of the 

 future iris. This is a mesoblastic formation forming the iris, 

 choroid and sclerotic. The cornea is still very thin, appa- 

 rently consisting as yet mainly of epiblastic structures. 



A single row of horizontally-flattened cells next appears 

 just internal to the outer molecular layer. The inner molec- 

 ular layer becomes broader, and soon shows a distinct, fine, 

 dark, median band. The ganglion-cells further increase in 

 size. The pigment develops rapidly, and soon sends down 

 processes between the segments of the rods and cones. An 

 external limiting membrane now becomes visible. The cornea 

 also shows intrusion of mesoblastic elements into its central 

 part, and a further thin process of mesoblast lines it inter- 

 nally, forming Descemet's membrane. In the marine 

 Teleosteans the vitreous chamber is late in developing, but 

 shows itself about the stage now reached. 



