544 ^- ^^- Hegner 



The cytoplasm of the beetle's egg is not made incapable of 

 development by centrifugal force, since an embryo may be pro- 

 duced after a profound change in its arrangement. Gurwitsch 

 ('04, '05) concludes, from his experiments upon the eggs of am- 

 phibians and echinoderms, that no vital structure of the cytoplasm 

 is destroyed by the forcible passage of yolk granules through it. 

 Morgan's conclusions from his experiments with the eggs of Rana 

 palustris are, on the contrary ('02, p. 306), that "The most impor- 

 tant effect, how^ever, of a strong centrifugal force is the direct injury 

 to the protoplasm of the lovv^er hemisphere of the egg." 



The nuclei. In C.B. 4, d (Fig. 10) one nucleus v^as found near 

 the inner end of the egg. The nuclei of eggs which are in late 

 cleavage stages rise toward the lighter pole (compare Figs. 17 and 

 18). In later stages the nuclei of the blastoderm are not affected, 

 but the vitellophags move through the yolk toward the inner end. 

 In every case the nucleus with its amoeboid accumulation of 

 cytoplasm moves as a whole, the nucleus remaining approxi- 

 mately in the center of the cytoplasmic mass. Lyon ('06) found 

 that the nucleus in the egg of the sea-urchin, Arbacia, is less dense 

 than most of the other constituents. In centrifuged eggs of As- 

 terias and Phascolosoma the nucleus is next to the lightest sub- 

 stance (Lyon '07) ; this is also the case in Hydatina senta (Whitney 

 '09, p. 135). In Paramecium caudatum the nucleus is heavier 

 than the endosarc and is driven to the outer end by the centri- 

 fugal force (McClendon '08). Similarly Andrews ('03) has found 

 that in seeds the nucleus is always of higher specific gravity than 

 the cytoplasm, cell sap and oil drops. 



When the membrane dissolves the nuclear sap escapes, leaving 

 the heavier chromatin behind. Thus we find that the spindle 

 does not rise toward the lighter end of the egg. Lillie ('06, p. 

 1 79) found in Chaetopterus that the maturation figure is not moved 

 by centrifugal force, but is usually fixed at the periphery. Some- 

 times, however, it was torn loose (p. 184), when it moves as a 

 whole, the chromosomes and spindle never being separated by the 

 centrifugal force. The same is true of Hydatina senta (Whitney 

 '09, p. 155). Morgan ('08, p. 446) makes the following state- 

 ment after a study of the effects of centrifugal force upon the eggs 



