114 



GRAVITY AND PROTOPLASM 



[Ch.V 



ova in whicli 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 Heeeick ('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 lohster 

 hardened with its dorsal surface (23) upper- 

 most. The nucleoli lie against the ventral 

 surface of the nucleus. Magnified 50 diame- 

 ters. (From Heeeick, '95.) 



Fig. 23. — Section through the 

 nucleus of a young ovum 

 (J 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 Hbr- 

 EICK, '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. CONTEOL OF THE DiEECTION OP LOCOMOTION BY 

 GeAVITY — GrBOTAXIS * 



The control of the movements of Protista has been investi- 

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

 .Euglena and Chlamidomonas ; Adbehold ('88), who studied 



* So called by Schwarz ('84, p. 71). 



