390 



CHARLES H. SPURGEON. 



show considerable shrinkage of the connective tissue and after 

 some months the cuticula shrinks. Dehydration and embedding 

 also cause further shrinking. See photomicrographs A and B. 

 The optic stalk may then approach the conical form observed 

 by Packard and by Parker. 



The cuticula of the eyes of C. setosus and C. pellucidus is 

 usually smooth. Sometimes it is wrinkled by the fixer or pre- 

 servative. These wrinkles may have led Newport to the con- 

 clusion that it is faceted. The main points of interest concerning 

 the cuticula are; first, it is thinnest in that part of the optic stalk 

 occupied by the cells of the vestigeal eye; second, it is laminated. 



mu5 



Fig. 6. Horizontal section of the eye of C. setosus, with the cuticula removed 

 Made from a section near the one shown in photomicrograph A. 



Sometimes the cuticula in C. setosus and in C. pellucidus is from 

 two to three times as thick on the sides as it is on the anterior end 

 or retinal region of the optic stalk. My photomicrographs A 

 and B and Fig. 6 show that the cuticula is thinnest in the retinal 

 region. According to Gilbert the average thickness of the retinal 

 cuticula is 3.41 mm., while that of the sides of the stalk is 12.41 

 mm. This is quite contrary to Parker's statement that "The 

 optic stalk is covered with a cuticula which is of uniform thick- 

 ness." 



It will be recalled that the cuticula is secreted by the hypo- 

 dermis. The hypodermis which is of ectodermal origin becomes 

 differentiated into the visual organ in Arthropoda. This dif- 

 ferentiation consists of the thickening and invagination of the 

 hypodermis. 



