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 



i 



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.) 



FIQ. 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). 



