114 



GRAVITY AND PROTOPLASM 



[Cn.V 



ova in which the yolk sinks to the lower pole and the proto- 

 plasm floats on top, in whatever position the egg may be held. 

 This fact undoubtedly has an important effect upon develop- 

 ment, as we shall see later. 



Of the specifically heavier bodies above referred to, the 

 nucleolus is a striking example, as HERRICK ('95) has recently 



shown. Thus, when the 



ovary of a lobster is 



killed, the nucleoli of 

 all the nuclei are found 

 in contact with that part 

 of the nuclear membrane 

 which was the lowest at 



FIG. 22. Section through the ovary of a lobster 

 hardened with its dorsal surface (D) upper- 

 most. The nucleoli lie against the ventral 

 surface of the nucleus. Magnified 50 diame- 

 ters. (From HERRICK, '95.) 



FIG. 23. Section through the 

 nucleus of a young ovum 

 ( mm. in diameter) showing 

 the nucleolus, which has, ap- 

 parently, caused a distention 

 of the nuclear membrane by 

 the pressure of its own weight. 

 Arrow shows the direction of 

 the earth's centre. Magnified 

 248 diameters. (From HER- 

 RICK, '95.) 



the moment of killing (Fig. 22). The weight of the nucleolus 

 is relatively so great as sometimes to cause a depression in the 

 part of the nuclear membrane upon which it rests (Fig. 23). 



3. CONTROL OF THE DIRECTION OF LOCOMOTION BY 

 GRAVITY GEOTAXIS * 



The control of the movements of Protista has been investi- 

 gated chiefly by four naturalists : SCHWARZ ('84), who studied 

 Euglena and Chlamidomonas ; ADERHOLD ('88), who studied 



So called by SCHWARZ ('84, p. 71). 



