-J44 J. GRAHAM KERR. 



face turned towards the lens, as well as in the part of each 

 cell which adjoins its neighbour. The nucleus occupies the 

 greater part of the body of each cell, and the part of the cell 

 behind it is free from pigment. 



In the retina itself the commencement of dilfei'entiation is 

 seen in the fact that its central and inner portion is becoming 

 free from nuclei, the cell-bodies in this region being composed 

 of clear transparent protoplasm. 



The development of the rods is the last important thing to 

 take place in the development of the retina. The precise 

 period of their appearance is, however, extraordinarily vari- 

 able. While as a rule they do not appear till about Stage 35, 

 I have met with them at least partially developed as early as 

 Stage 32. 



The appearance of the rod is heralded by the development 

 here and there in the protoplasm of a visual cell of a small 

 spherical droplet apparently of a fatty nature, staining black 

 with osmic acid. As a rule these appear about Stage 35. 



In the case of a visual cell which is about to develop a rod, 

 the fatty droplet just mentioned increases in size, and comes 

 into close contact with the external limiting membrane. 

 As the droplet increases in size the membrana limitans be- 

 comes pushed out over it to form a hemispherical bulging. 

 This bulges out more and more, until eventually it foruis a 

 deep pocket in which lies the fatty vacuole and a small 

 quantity of protoplasm round it. As the vacuole continues 

 to increase in size after it has passed into the pocket the 

 latter often assumes a pear shape, being narrower at its base 

 of attachment to the limiting membrane. This is shown in 

 fig. 11, J to L. 



The nucleus may project more or less into the pocket, and 

 its peripheral end squeezing against the vacuole often 

 pushes in its inner wall, so as to convert the sphere into a 

 concavo-convex lens. 



The rudiment of the rod appears as a conical projection of 

 protoplasm beyond the vacuole. This condition is illustrated 

 l)y fig. 1, J. The cuticuhir boundary of the rod rudi- 



