382 



THE PERIPHERAL NERVOUS SYSTEM 



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 proxi- 

 mal layer of the iris and blends intimately with the pigment layer in this region, 

 its cells also becoming heavily pigmented (Fig. 379). 



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

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

 irmer fibrous layer may be distinguished in 12 mm. embryos (Fig. 377). These 



correspond to the cellular 

 layer (ependymal and mantle 

 zones) and marginal layer of 

 the neural tube. In fetuses 

 of 65 mm. (C R) the retina 

 shows three layers, large 

 ganglion cells having mi- 

 grated in from the outer 

 cellular layer of rods and 

 cones (Fig. 380). In a fetus 

 of the seventh month all the 

 layers of the adult retina 

 may be recognized (Fig. 381). 

 As in the wall of the neural 

 tube, there are differentiated 

 in the retina supporting tis- 

 sue and nervous tissue. The 

 supporting elements, or fibers 

 of Mailer, resemble epen- 

 dymal cells and are radially arranged (Figs. 380 and 381). 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, which are at first miipolar (Fig. 381). 

 Internal to this layer are layers of bipolar and multipolar cells. The inner layer 

 of multipolar cells constitutes the ganglion cell layer. Axolis from these cells form 

 the inner nerve fiber layer of optic fibers. These converge to the optic stalk, and, 

 in embryos of 15 mm., grow back in its wall to the brain. The cells of the optic 

 stalk are converted into neuroglia supporting tissue and the cavity of the stalk is 

 gradually obhterated. The optic stalk is thus transformed into the optic nerve 

 (cf. p. 358). 



Pigment layer 

 Rods and Cones 



Outer nuclear layer 



Outer reticular layer 

 Inner nuclear layer 



Inner reticular layer 

 Ganglion cell layer 



Nerve fiber layer 

 Fibers of Miiller 



Internal limiting membrane 



Fig. 381. — Section through the pars optica of the retina 

 from a seven months' fetus. X 440. 



