THE SIZE OF ORGANISMS 187 



end it consists of a large vesicle of achromatic substance in which the 

 chromatin usually exists as scattered granules. At the next division 

 some of these granules form chromosomes and all the rest of the nuclear 

 content is liberated into the cell body, to be again absorbed by the 

 daughter nuclei during the succeeding rest period. There is thus a 

 sort of diastole and systole of the nuclear vesicle during every division 

 cycle of a cell, achromatin being taken up by the nucleus during its 

 growth and liberated again into the cell body during its division. 



In different cleavage cells of Crepidula plana, when the yolk is elimi- 

 nated from consideration, the nucleus-plasma ratio varies from 1 : 0.37 

 to 1 : 14.5 ; that is, the volume of the actual protoplasm in certain cells 

 may be only one third the volume of the nucleus, or in other cells it may 

 be fourteen times that volume, depending largely upon the length of 

 the resting period. 



In general the size of a nucleus is directly proportional to the volume 

 of the general protoplasm in the cell, to the length of the resting period, 

 and in cases of abnormal or irregular distribution of chromosomes, to 

 the number and volume of the initial chromosomes which go to form 

 the nucleus. The inciting cause of cell division is not to be found in 

 departures from a normal nucleus-plasma ratio, which is a result rather 

 than a cause of the rate of cell division, but rather in the coincidence of 

 certain metabolic phases in nucleus, centrosome and protoplasm. 



If the growth period of the nucleus is very long, the greater part of 

 the protoplasm may be taken into the nucleus, as in those cleavage cells 

 in which the nuclear volume is about three times as great as that of the 

 protoplasm outside of the nucleus ; if the growth period of the nucleus 

 is short, the nucleus remains correspondingly small. If nuclear division 

 is prevented by hypertonic solutions or by decreased oxygen tension, the 

 nuclei may grow to an enormous size until they contain the greater part 

 of the cell protoplasm. 15 



In certain stages of the division cycle it is possible by the use of 

 hypertonic solutions to prevent the daughter chromosomes from absorb- 

 ing achromatin, and in such cases these chromosomes form small, 

 densely chromatic nuclei, while the achromatin may be gathered into 

 one or many vesicles. In other cases, the chromatin may be caused to 

 contract and to squeeze out the achromatin. The latter case is similar 

 to that which takes place normally in the formation of a spermatozoon 

 from a spermatid, where there is a condensation of the chromatin of the 

 spermatid nucleus and a squeezing out of the achromatin; this diminu- 

 tion of the nucleus is coincident with the transformation of the proto- 

 plasm of the spermatid into differentiation products. A similar thing 

 happens in superficial epithelial cells which are undergoing keratiniza- 



15 Conklin, ' ' Experimental Studies on Nuclear and Cell Division, ' ' Jour. 

 Acad. Nat. Sci. Phila., 15, 1912. 



