662 



THE EYE 



four to five times as stout as that of the rod cell. These processes carry 

 apically a diplosome, surrounded by a lighter cytoplasmic halo. From each 

 member of the diplosome a fiber-process grows out, the one passing towards 

 the elongating distal, the other towards the proximal, pole of the cell. The 

 distal process thus becomes enveloped by an extension of the cell cytoplasm, 

 the two constituents, fiber and investing cytoplasm, uniting in the forma- 



* /! 



W) 



$e<j . 



- 



,., 







mat 



J5 



FIG. 561. Two EARLY STAGES IN THE DEVELOPMENT OF THE ROD AND CONE 



VISUAL CELLS IN THE CHICK. 



A, from 12-day embryo, showing the visual cell buds containing mitochondria; 

 L.e., external limiting membrane; C.V., visual cell; R.e., external reticular layer; 

 C.p., bipolar cells; B, from 1-day -old chick, showing two complete rods and one 

 cone, both elements containing a large lipoid spherule (G) and mitochondria. The 

 cone contains an ellipsoid (E); seg. int., internal segment; seg. ext., external segment. 

 (Leplat.) X about 2000. 



tion of the outer segment of the rods and cones. Seef elder ("Atlas zum 

 Entwickslungsgeschichte des Menschlichen Auges," Leipzig, 1914) has re- 

 cently confirmed the essential points in the earlier descriptions of rod and 

 cone differentiation by Leboucq (1909) and by Magiott (1910). Leplat 

 (Anat. Anz., 45, 8, 1913) has investigated the role of the mitochondria in 

 connection with this process. He describes the migration of the 'plasto- 

 somes' into the external segment where they become chemically altered 

 and disappear as such in their contribution to the homogeneous cytoplasmic 

 sheath of the centrosomal filament of the external segment. Leplat inclines 

 to regard the transverse striation of this segment and its cleavage into 



