2 THE CHROMOSOMES 



it, it becomes indented at the point where the pressure 

 is applied ; if the pressure is released it regains its 

 former shape ; if the pressure is increased it can 

 eventually be punctured. ^^ The shape of the nucleus 

 undoubtedly depends in part on the properties of 

 the nuclear membrane. Most nuclei are approxi- 

 mately spherical, but many are ovoid. Those of 

 many Wrtebrate leucocytes are in the form of a long 

 strand with periodic enlargements (Fig. la) while 

 those of the secretory cells of many insects are very 

 irregularly branched (Fig. 16).^^^ In all these cases 

 of non-spherical nuclei the nuclear surface is very 

 large relative to its volume, and it has been assumed 

 that this is connected with the process of secretion ; 

 but many secretory cells (such as the salivary gland 

 cells of liiptera) have approximately spherical nuclei. 

 Some unusual nuclei do not consist of a single body 

 at all, but of a number of separate vesicles, each con- 

 taining a single chromosome and a certain amount of 

 nuclear sap inside a separate membrane (Fig. Ic). 

 In one case the sex chromosome is enclosed in a 

 separate membrane from the main nucleus (Fig. Id). 



The nuclear sap is usually a clear fluid ; its vis- 

 cosity has been determined in one case to be about 

 twice that of the water, ^^ and this is probably 

 typical of most nuclei ; in some, however, the nuclear 

 sap may be a solid gel. The amount of nuclear sap 

 relative to the volume of the chromosomes varies 

 enormously from one type of nucleus to another. 

 Thus in the micronucleus of Ciliates and in the sperm- 

 head nuclei of many animals there is practically no 

 nuclear sap ; on the other hand, the total volume 

 of the young oocyte nuclei of birds (diameter up to 

 100 fj) may be 200,000 times that of the chromosomes 

 at metaphase. 



The chromosomes may be, and usually are, in- 

 visible in living nuclei during the resting stage. 2*- 

 60. 100 jj^ some plant nuclei, however, and also in a 

 few animal nuclei, fine threads can be seen in the 



