1902.] in the Chick after the Formation of the Optic Cup. 85 



therefore its first appearance is not due to the growth of the pro- 

 cesses from these cells into it. The three streaks which develop in the 

 internal molecular layer are due to a denser texture of the myelo- 

 spongium at these places, on either side of which the fine arborisations 

 from the cells in the ganglion layer and internal nuclear layer tend to 

 accumulate. 



The first appearance of the external molecular layer is explained as 

 follows : — The myelospongium fibres bifurcate near their outer ends 

 and neighbouring branches fuse with one another, so that the external 

 molecular layer has at first an irregular outline. Later on, when the 

 retinal wall becomes thinner, then this layer has a straighter outline. 

 The persisting radial fibres of the myelospongium become the fibres of 

 Miiller. 



In the internal nuclear layer, three kinds of cells can be dis- 

 tinguished at the 12th day of incubation: 1st, a single row of cells 

 with clear nuclei, which lie next to the external molecular layer (basal 

 cells) ; 2nd, several layers of small bipolar cells with external and in- 

 ternal processes passing into the external and internal molecular layers 

 respectively ; 3rd, a set of cells, usually arranged in three rows and 

 giving off processes into the internal molecular layer (amacrine cells). 

 From the 10th to the 12th day the ganglion cells spread out to form one 

 layer, and the bipolar cells of the internal nuclear layer also spread out 

 so that the retina becomes thinner. At the 10th day two kinds of cells 

 can be recognised in the external nuclear layer — 1st, cells which extend 

 from the external limiting membrane to the external molecular layer, 

 and are of the same breadth throughout (the rod cells) ; 2nd, cells which 

 taper towards the external molecular layer (the cone cells). The 

 rods and cones develop on the 12th day as outgrowths from these 

 cells. The rods, however, appear a little earlier than the cones, and 

 form globular projections which soon become flask-shaped. One cone- 

 element appears usually between two flask-shaped rod-elements, and 

 are therefore forced to assume a cone shape, but from them a fine 

 filament is forced between the rod-elements and forms a minute 

 spherical swelling immediately beyond. In most of the rod and cone 

 cells a spherical body can be seen lying between the nucleus and the 

 external limiting membrane on the 14th day. The rods and cones 

 increase in length, due to protrusion of more and more protoplasm from 

 the rod and cone cells, and this spherical body (the rod and cone 

 ellipsoid) is borne along with the later protrusions of protoplasm 

 and comes to occupy the inner segments of the rods and cones. 

 Thus the outer segments of * the rods and cones are the first to be 

 protruded and lastly the inner segments. The oil globule develops 

 in the protruded portion of the cone-element on the 15th day, and 

 becomes coloured red or yellow on the 17th or 18th day. When 

 first developed the rods about equal the cones in number, whereas 



