22 



cytoplasm has still a large share in the interpretation of 

 these problems.^ The acquirement of new characters and 

 the vexed questions of their inheritance, or non-inheritance, 

 with the Mendelian and many other considerations, all 

 centre around the nucleus for solution. 



SUMMARY. 



The cell-theory of Schleiden and Schwann gave 

 the greatest impetus to microscopic investigations, particu- 

 larly those centering around- the nucleus ; discovery of 

 the nucleus and further research established it to be the 

 essential factor of the cell ; it was supposed to originate 

 within and from the cytoplasm, the nucleolus first appearing 

 and the nucleoplasm precipitating around it ; proof 

 accumulated that there was no de novo formation of nuclei, 

 and that every nucleus in the organism descended from the 

 fertilised-egg nucleus. 



The Nucleus is of diverse shapes, complex structure, 

 and shows different physical and chemical characters during 

 stages of its history and development. 



The Nucleoplasm. — Its structural basis is in the 

 form of a linin reticulum, similar to the network found in 

 the cytoplasm ; both are supposed to be of the same 

 material, and comminojle during division of the nucleus. 

 The nuclear-sap, or nutrient material, is enclosed within the 

 meshes, and may show the alveolar appearance of Biitschli. 



Chromatin. — -The staining material of the nucleus is 

 deposited along the specialised linin walls of the reticulum 

 of the resting nucleus, the spirem thread in mitosis and in 

 the chromosomes. It is usually in the form of fine 

 granules — chromomeres or ids, karyosomes, &c., or it may 

 appear homogeneous. The granules lie in bars or bands 

 across the fibrils, and are separated into more or less equal 

 pairs, with the longitudinal split in the spirem. 



