168 



The conditions of the eye now reached belong either to the larva 

 or to the embryo, at the point of hatching. I purposely omit here a 

 description of the development of the gangliogen, and of the histology 

 of the adult eye, but wish to emphasize the following points: 



1) The optic disc is at first a unicellular layer of ectoderm: 



2) This disc becomes thickened by emigration of cells from the 

 surface ; by delamination, and probably also by the addition of cells 

 from the yolk (inner layer as distinguished from the outer ectoderm). 



3) The lobular mass of ectoderm thus formed (optic lobe), is diffe- 

 rentiated into two parts, separated by a structureless membrane (base- 

 ment membrane). The latter is probably secreted by the ectoderm cells 

 along the line of division. 



4) From the outer portion or retinogen, which is at first a single 

 layer, the retina is developed. The rest of the lobe (gangliogen ' , gives 

 rise to the ganglia and parts of the eye below the basement mem- 

 brane. 



.5) Retinulae, retinophorae and corneal cells are differentiated 

 ectodermal elements belonging to the retinogen. 



6) The retinophorae are prolonged inwards with the cone but a 

 short distance, and do not enter the retinular bundle. 



7) There is no swollen pedicel, and nothing answering to pedicel, 

 »rhabdom«, or «spindle« has been detected. If anything corresponding 

 to it exists, it must be the inner solid cone of the retinular bundle, 

 which is a product of the retinulae themselves. 



8) No invagination or cavities of any kind occur in the develop- 

 ment of this eye. 



9) The ommatidium or element of the compound eye consists of 

 13 cells disposed in 3 layers, as follows. 



a.) corneal layer — 2 cells. 



b) retinophoral layer — 4 cells. 



c) retinular layer — 7 cells. 



10) No nerve fibres have ever been detected in the crystalline cones. 

 — (The section method Avas used entirely. The eggs and larvae were 

 preserved in picrosulphuric acid, Perenyi's fluid, and alcohol. Ì 



Tt is undoubtedly true that the development of Alpheus has under- 

 gone secondary changes in the course of its history, but there is no 

 reason to suppose that it has diverged in this respect more than related 

 species ofMacroura. INluchless would webe led to think that it was pecu- 

 liar in the development of its eye. Studies upon other Macroura [llippa, 

 Palaemonetes) have led me to suspect that the method here described 

 is the common one 



