MUSEUM OF COMPARATIVE ZOOLOGY 207 



nenberg's haematoxylin. The outline figure on thin paper (Fig. 3a) h 

 taken from the section directly external to the one just described. X 475. 



Fig. 4. Posterior face of a part of a transverse section of the retina described in 

 Fig. 2. X475. 



Fig. 5. A retinal cell isolated in ■^^% solution of chromic acid, and examined in a 

 mixture composed of equal parts of water and glycerine. X 475. 



Fig. 6. A pigment cell isolated and examined in the same manner as that shown 

 in Fig. 5. X475. 



Fig. 7. A retinal cell with an attached pigment cell. The pigment was removed 

 with a solution of potassic hydrate, and the cell was isolated and stained 

 in Grenacher s alcoholic borax-carmine. Examined in a mixture of 

 glycerine and water. X 475 



Fig. 8. Tlie posterior lateral portion of a horizontal section of a retina seen from 

 the dorsal side. Partially depigmented with a solution of potassic hy- 

 drate, and subsequently stained with Czoker's cochineal. In several 

 places the lentigen has been artificially ruptured. X 475. 



Figs, y to 17 represent the structure of the median eyes in young scorpions. 



Fig. 9. The posterior face of a transverse section of the right retina in a young 

 scorpion about the age at which it leaves the mother's back. Depig- 

 mented with potassic hydrate ; stained in Czoker's cochineal. X 195. 



Fig. 10. The left face of a section through the right eye parallel to the sagittal 

 plane. Pigment unchanged; stained in Grenacher's alcoholic borax- 

 carmine. This specimen was taken about the time of birth. X 195. 



Fig. 11. Right face of a section from the sagittal plane of an individual somewhat 

 younger than that seen in Fig. 10. In this section the neck of the in- 

 vagination (col.) is seen to reach almost to the retina. In the sections 

 on either side of this the neck appears much reduced, and at no point 

 does it unite with the retina. X 195. 



PLATE III. 



Fig. 12. The right face of a section almost in the sagittal plane of an embryo. The 

 lower part of this section was exactly in the median plane. The upper 

 part was somewhat to the right of that plane. Pigment unaffected ; 

 stained in Grenacher's alcoholic borax-carmine. X 195. 



Figs. 13 to 17 represent the dorsal faces of a series of horizontal sections in the re- 

 gion of the median ej-es. In all the pigment is unchanged, and all have 

 been stained with Grenacher's alcoholic borax-carmine. The region of 

 the eye extends through forty-four sections. X 195. Beginning with the 

 most ventrally situated and passing dorsally, Fig. 13 represents the 

 seventh. It will be noted that the sections are not strictly horizontal, 

 but that they dip slightly to the left ; consequently in Fig. 13 the wall 

 of the pocket is cut on the right in the thickened or retinal region, but 

 on the left nearer the orifice of the pocket. 



Fig. 14, the fourteenth section, shows the cavity of involution just before it is 

 divided into a right and left compartment. 



Fig. 16, the twenty-third section, shows the cavity divided. 



Fig. 16, the thirty-first section, shows the right compartment reduced in size. 



