408 NATHAN FASTEN. 



structure disappears during the next stage." In a later paper 

 (Wilson, '15), on the Lernaeopodidse, this same author states 

 the following: 'The eye in this whole family is extremely rudi- 

 mentary, appears only for a short time during the development 

 stages, and then entirely disappears." In Salmincola edwardsii, 

 which also belongs to this family of Lernaeopodidae, the eye is 

 well developed and resembles to a marked degree the visual 

 organ of the free-living marine copepod Eucalanus elongatus 

 Dana, worked on by Esterly ('08). Furthermore, during the 

 metanauplius stage, the eye of Salmincola ediuardsii makes its 

 appearance and attains its full development in the free-swimming 

 larval form, the so-called first copepodid stage. 



METHODS. 



Entire mounts as well as sections of the larvae were used for 

 this study. Various fluids, such as Bouin's, Gilson's and 5 per 

 cent, corrosive-acetic were tried for fixation, but the last-men- 

 tioned reagent yielded the best results and was used almost 

 exclusively. 



The entire mounts were made in the following manner. The 

 organisms were placed in 5 per cent, corrosive-acetic fluid for 

 ten minutes or longer, and then they were washed in many 

 changes of water. After this they were run up through the 

 various grades of alcohol, being left about ten minutes in each, 

 and finally they were cleared in xylol and mounted in balsam. 

 Larvae thus treated yielded beautiful results, showing little 

 change from the normal condition. Those which were allowed 

 to remain in the fixative longer than ten minutes, generally 

 had most of their pigments dissolved out, thereby making it 

 possible to obtain a fine view of the external structure of the eye. 



The larvae to be sectioned were also fixed in the corrosive- 

 acetic mixture. After dehydrating, clearing and infiltrating, the 

 organisms were permanently imbedded in paraffine for sectioning. 

 The sections were cut from 3-6 IJL in thickness, in frontal, trans- 

 verse and sagittal planes, and were stained in Heidenhain's 

 iron-haematoxylin, with a counterstain of acid-fuchsin or eosin. 

 These sections were very helpful in determining the internal 

 structure of the eye. 



