DEVELOPMENT OF THE SENSE ORGANS 



379 



nervous retina proper. The line of demarcation between the pars optica and the 

 pars caeca is a serrated circle, the ora serrata. The blind portion of the retinal 

 layer, the pars caeca, with the development of the ciliary bodies is differentiated 

 into a pars ciliaris and pars iridis retina. The former, with a corresponding zone 

 of the pigment layer, covers the ciliary bodies. The pars iridis forms the 

 proximal layer of the iris and blends intimately with the pigment layer in this 

 region, its cells also becoming heavily pigmented (Fig. 360). 



The pars optica, or nervous portion of the retina, begins to differentiate prox- 

 imally, the differentiation extending distally. An outer cellular layer and an 

 inner fibrous layer may be distinguished in 12 mm. embryos (Fig. 358). These 

 correspond to the cellular layer (ependymal and mantle zones) and marginal 



Cone cell 

 Rod cell 



Rod cell 



Fiber of 

 Mueller. 

 Atnaerine 

 cell 



Ganglion 

 cell 



Optic - 

 fibers 



External limit- 

 ing membrane 



Layer of rod 

 and. cone cells 



\ Ganglionic 



layer 



^ Fibrous layer 



Internal limit- 

 ing membrane 



Fig. 361. — Section of the nervous layer of the retina from a 65 mm. embryo. The left portion of the 

 figure shows diagrammatically the cellular elements of the retina according to Cajal. X 440. 



layer of the neural tube. In embryos of 65 mm. the retina shows three layers, 

 large ganglion cells having migrated in from the outer cellular layer of rods 

 and cones (Fig. 361). In a fetus of the seventh month all the layers of the adult 

 retina may be recognized (Fig. 362). As in the wall of the neural tube, there are 

 differentiated in the retina supporting tissue and nervous tissue. The supporting 

 elements, or fibers of Mueller, resemble ependymal cells and are radially arranged 

 (Figs. 361 and 362). Their terminations form internal and external limiting 

 membranes. 



The neuroblasts of the retina differentiate into an outer layer of rod and 

 cone cells, the visual cells of the retina (Fig. 362). Internal to this layer are layers 

 of bipolar and multipolar cells. The inner layer of multipolar cells constitutes 

 the ganglion cell layer. Axons from these cells form the inner nerve fiber layer 



