Parker — Lobster Eye. 



PLATE II. 



Figs. 26 to 36 deal with the histology of the lobster's eye ; Figs. 37 to 39, with its 



development. 



Fig. 26. Vertical section through that portion of the eye-stalk where the transi- 

 tion from the undifferentiated hypodermis to the ommateum is accom- 

 plished. The open space x is due to shrinkage. X 31. 



" 27. A group of the four cone-cells of an ommatidium and the attached cor- 

 neal hypodermis. Isolated and studied in Muller's fluid. X 365. 



" 28. Proximal end of a group of four cone-cells where they separate as fibres 

 to pass around the rhabdome. Isolated and studied in Muller's fluid. 

 X 365. 



" 29. Transverse section of the basement membrane. The distal face is upper- 

 most; the proximal face below. The section is taken in a plane 

 which would be represented in Fig. 21 by the line z y. x 575. 



" 30. A rhabdome and its seven surrounding proximal retinulse. At the distal 

 end the free tips of the seven retinulae can be seen. At the proximal 

 end the rhabdome and the four groups of retinulae which pass through 

 the basement membrane are visible. Isolated and studied in chromic 

 acid sVj%. X 200. 



" 31. An individual proximal retinula. Isolated and studied in chromic acid 

 io%- X200. 



" 32. Transverse section of a rhabdome and its surrounding cells. The plane 

 of section is about half-way between the middle and distal end of the 

 rhabdome. Completely depigmented with potassic hydrate. X 460. 



" 33. Oblique section of a rhabdome from the same series of sections as Fig. 32. 

 The upper end of the figure is distal; the lower, proximal. X 460. 



" 34. Transverse section of a rhabdome and its surrounding retinulae. The 

 nervous axis of each retinula is distinctly stained. Completely de- 

 pigmented ; stained with Kleinenberg's alum-haematoxylin. x 460. 



" 35. Longitudinal section of a bundle of nerve-fibres extending through the 

 basement membrane toward the optic ganglion. Depigmented ; 

 Weigert's haematoxylin. x 575. 



" 36. Optic nerve-fibre. Isolated and studied in Muller's fluid, x 575. 



" 37. Superficial view of a left optic lobe. Enough of the right lobe is drawn 

 to indicate the position of the median plane (x y). x is anterior; y is 

 posterior. Stage A (see page 2). X 280. 



" 38. Posterior face of a section from a right optic disk cut transversely to the 

 longitudinal axis of the embryo (see page 34). At x is an angle formed 

 by the growth of the retinal cells over the undifferentiated ectoderm. 

 Stage A. x 280. 



" 39. A section from the optic disk of an embryo in stage B. The plane of 

 cutting corresponds to that in Fig. 38. In this figure x indicates, as 

 in the preceding one, the angle between the undifferentiated ectoderm 

 and the growing retina. X 280. 



