652 MANUAL OF HISTOLOGY. 



onic body. The first trace is seen in the form of a hollow stalked process 

 of the anterior portion of the brain, the primary optic vesicle, which is 

 destined in its farther development to transformation into the retina and 

 pigmentary epithelium of the choroid, while the pedicle becomes the optic 

 nerve. We have already seen that the lens springs from that portion of the 

 corneous germinal layer which lies over the rudimentary eye, growing in- 

 wards in the form of a thick-walled hollow sphere (comp. 161). Besides 

 the latter, and underneath it, there presses into the primary optic vesicle 

 the vitreous humour, a production of the middle germinal layer, i.e., of the 

 dermal tissue of the head. These two organs thus double in the optic vesicle 

 into itself, after which, as in the serous sacs, two lamellae of formative 

 cells may be distinguished, a thicker internal one, the future retina, and 

 an external thin layer destined to form the pigmentary epithelium. "We 

 now have before us what is known as the secondary optic vesicle. The 

 fibrous layer of the choroid, the ciliary body and muscle, the iris, sclerotic, 

 and cornea, all spring from the adjacent portions of the middle germinal 

 layer of the rudimentary embryonic head. 



That the whole retina, with the exception of the pigmentary epithelium, 

 takes its rise from the internal lamella of the secondary optic vesicle, is a 

 fact observed long ago by Remak and Koellilter, which has been con- 

 firmed more recently by Babiichin and Schultze. The first elements to 

 be recognised, according to Babuchin, are the rudiments of the fibres of 

 Muller, in the form of fusiform cells, terminating at either end in the 

 limiting membranes (external and internal) which are formed from them. 

 After the fibres of Muller the ganglion cells are next developed, and 

 then the stratum fibrillosum. The molecular and inter-granular layers, as 

 well as that of the rods and cones, appear almost simultaneously ; the first- 

 mentioned layer, however, precedes the others somewhat in formation, 

 and the stratum bacillosum brings up the rear. The rods, cones, and 

 cone-bodies are produced, according to Babuchin, in the tadpole as pro- 

 cesses or outgrowths from cells. They thus constitute with their cells 

 ("granules" or " corpuscles") an inseparable whole. 



The development of the central portion of the retina, moreover, hurries 

 on in advance of that of the periphery (Babuchin, Schultze). 



A number of delicate hemispheroidal growths were recognised by 

 Schultze in the chick projecting from the outer surface of the membrana 

 limitans externa. These are stated by him to be developed later on into 

 rods and cones. 



The outer halves or members of the rods, which also undoubtedly 

 belong to the internal wall of the secondary optic vesicle, are formed 

 much later than the inner members. 



In connection with the late development of the rods there is a point of 

 much interest mentioned by Schultze. Animals which are born with 

 closed eyes as, for instance, cats and rabbits do not at birth possess 

 those retinal elements, while in man and the ruminants they are already 

 fully developed when born. 



The external layer of the so-called secondary optic vesicle is formed by 

 a single row of perpendicular, and originally elongated, but later more or 

 less cuboid cells, which become eventually flattened. This is gradually 

 transformed into pigmentary epithelium, as has been already remarked, 

 by the absorption of molecules of melanin. This layer is generally 

 reckoned among the laminae of the choroid, whereas it belongs in reality 

 to the retina. 



