112 



INTRODUCTION TO CYTOLOGY 



a finely reticulate stroma in which an oily red pigment with many of the 

 characteristics of hsematochrom is held in the form of minute droplets or 

 granules (Schilling 1891; Klebs 1883; Franze 1893; Wager 1900; Wollen- 

 weber 1907, 1908) (Fig. 41, D). As shown by the careful researches of 

 Franze, the stroma may also bear one or more refractive inclusions, 

 which in the Chlamydomonadacese and Volvocacese consist of starch, and 



in the Euglenoidese of paramylum (Fig. 41, 

 E). These inclusions were thought by 

 Franze to increase the sensitivity of the eye- 

 spot by concentrating the light at certain 

 points. 



The eyespot of the zoospore of Cladophora 

 (Strasburger 1900) appears to arise as a 

 swelling of the plasma membrane, and consists 

 of an external pigmented layer beneath which 

 is a lens-shaped mass of hyaline substance 

 (Fig. 41, B). InGonium and Eudorina (Mast 

 1916) the lens-shaped portion lies outside with 

 the cup-shaped opaque portion beneath it 

 (Fig. 41, A), an arrangement strongly sug- 

 gesting the primitive eyes of certain higher 

 organisms. In neither portion could any finer 

 structure be detected. Mast has shown that 

 the orientation of the colony is brought about 

 through changes in the intensity of the light 

 falling upon the light-sensitive substance. As 

 the unoriented swimming colony rotates on 

 its axis, those zooids turning away from the 

 light have the hyaline portion of their eye- 

 spots shaded by the opaque cup; this sudden 

 reduction in the amount of light energy 

 received brings about an increase in the 

 activity of the flagellar of those zooids, with 

 the result that the colony as a whole turns 

 more directly toward the .source of light. 



In Euglena viridis the morphological con- 

 nection between the eyespot and the motor apparatus is particularly 

 close. Here Wager (1900) has shown that the eyespot, which is a 

 discoid protoplasmic body containing a layer of large pigment droplets, 

 is situated at the surface bounding the oesophagus in close contact with a 

 swelling on one of the basal branches of the flagellum (Fig. 41, C). 



In general it may be concluded that the eyespot in some cases bears in 

 its structure, and to a certain extent in its evident function, such a close 

 resemblance to the ordinary plastid that a relationship of some sort 



-Eyespots of various 

 types. 



A, zooid of Eudorina; e, eye- 

 spot. (From Mast, After Grave.) 

 B, zoospore of Cladophora, 

 (After Strasburger, 1900.) C, 

 anterior end of Euglena viridis, 

 showing eyespot at surface of 

 oesophagus, and in front of it a 

 swelling on one root of the 

 flagellum; face view of eyespot 

 at right, showing pigment gran- 

 ules. (After Wager, 1900.) D, 

 eyespot of Euglena velata. 

 (After Franze, 1893.) E, eye- 

 spot of Trachelomonas volvo- 

 cina, with pigment granules 

 and crystalloid body. (After 

 Franze.) 



