287 



optic nerve, the one terminating in the visual or clear cells, the other in 

 the tactile cells lying above and in front of the visual cells. In a later 

 paper (»A Sketch of the Structure and Development of the Eye of Clep- 

 sine«, Zool. Jahrbb., Bd. 6), Prof. Whitman says: »If the optic branch, 

 whih is extremely short in Clepsine, should separate from the main nerve 

 just behind and below the eye, and enter the base of the eye, instead 

 of passing to the front , as it now does , we should have the relations 

 shown in Maier's figure. This resemblance would be essentially 

 complete, if the tactile cells were changed into visual cells. It seems 

 to me most probable that this is precisely what has happened in the 

 history of the Hirudo eye. 

 This accounts for the fact 

 that the upper anterior 

 part of the eye is not co- 

 vered by pigment, and 

 for the further fact that 

 the visual cells in this re- 

 gion are often spread out 

 rather loosely in front of 

 the proper eye cylinder. 

 It is this surplus of cells 

 that I should regard as 

 that homologue of the 

 tactile elements in the 

 Clepsine eye.« 



This supposed homo- 

 logy of structure of nerve 

 and tactile cells in Clep- 

 sine and Hirudo, led Prof. 

 Whitman to assign th« 

 subject of the development 

 of the Hirudo eye to me as a problem for the summer's work at the 

 Marine Biological Laboratory at Wood's HoU, in 1893. 



Before beginning the study of the development of the eye of Ma- 

 crobdella , which among American leeches , is most nearly related to 

 the European medicinal leech, I studied the adult eye of Clepsine and 

 Macrohdella. Almost the first sections of Macrohdella revealed the 

 two kinds of cells found in the eye of Clepsine , namely : tactile and 

 visual cells. I then obtained specimens oi Atilostomum, the Swedish 

 medicinal leech Hirudo medicinalis^ and a new species, found by Prof. 

 Whitman, near Worcester, Mass., and in all specimens found tactile 

 and visual cells in the adult eye. In other respects the eye follows 



18* 



ai. t^cKie tells 

 t. cutitît. 



