106 THE THIRD DAT. [CHAP. 



posterior becomes much thinner than the inner or anterior, 

 and by the middle of the fourth day is reduced to a single 

 layer of flattened cells (Fig. 31, p. Ch.). At about the 80th 

 hour its cells commence to receive a deposit of pigment, and 

 eventually form the so-called pigmentary epithelium of the 

 choroid ; from them no part of the true retina (or no other 

 part of the retina, if the pigment-layer in question be sup- 

 posed to belong more truly to the retina than to the choroid) 

 is derived. 



On the fourth day, the inner (anterior) wall of the optic 

 cup (Fig. 31, R) is perfectly uniform in structure, being 

 composed of elongated somewhat spindle-shaped cells, with 

 distinct nuclei. On its external (posterior) surface a distinct 

 cuticular membrane, the membrana limitans externa, early 

 appears. 



As the wall increases in thickness, these cells multiply 

 rapidly, so that the wall becomes several cells thick. 



The first indications of a division into layers are noticed 

 on the seventh day ; and on the eighth day a layer of 

 'granules' is very obvious. The granules, which are appar- 

 ently nuclei of cells, become on the tenth day distinctly 

 arranged into an inner and an outer layer ; and at about 

 the same time small processes, apparently outgrowths from 

 the outer granular layer, make their appearance on the 

 external surface of the membrana limitans externa. These 

 processes are the rudiments of the rods and cones. 



From the first they may be roughly divided into two catejrories, (i) those of 

 smaller, (2) those of larger diameter. Both kinds grow rapidly and in the tips 

 of both small highly refractive globules soon appear. The tliinner processes 

 are the cones, the thicker the rods. The cones remain for a long time tliinner 

 than the rods, but .shortly before the exclusion of the chick they increase rapidlj' 

 in diameter and soon after that occurrence are found to surpass the rods in 

 thickness. On the i8th day some of the globules in tlie cones become 

 red, on the 19th others become yellow, and very soon all the globules in tlie 

 cones acquire a distinct colour. The globules in the rods remain uncoloured. 

 Tiie rods and cones then are outgrowths through the membrana limitans externa, 

 from the inner wall of the optic cup or retina into the outer wall or pigment- 

 epithelium of the choroid. 



Kemak and some other investigators were of opinion that the outer wall of 

 the optic cup gave rise to the rods and cones as well as to the pigment-epi- 

 thelium. The observations however of M.a.xi^c\\u\tze, Arcldv Micros. Anat iv. 

 p. 239, supported by Babuchin, Wilrz. Nat. Zeitsch. iv. (1863) p. 71, and others. 

 Lave clearly shewn that Remak's views were erroneous. 



On the thirteenth day the molecular layer and the gan- 



