i NUCLEAR CONSTITUENTS 19 



is embedded in the linin, but a few cytologists (e.g. Gregoire and Wygaerts, 

 1904 ; Lundegardh, 1912) hold the view that there is no distinction between 

 the two substances, and that the common aspect of the nuclear substance 

 as consisting of a deeply staining material superimposed on a much 

 finer and more weakly staining framework is not due to any chemical 

 difference between the two parts, but simply to the fact that the stain is 

 retained by the coarser masses (chromatin) but not by the very fine 

 strands usually interpreted as linin. This view, however, seems to have 

 little in its favour, and there are very great difficulties in the way of 

 accepting it. For instance, in the prophase the young chromosomes are 

 often connected by numerous fine transverse unstained threads (linin) 

 (Figs. 3, D, 16, G). The chromosomes are all approximately of the same 

 thickness, and the transverse threads, which are much finer than the 

 chromosomes, are also approximately equal in thickness to one another. 

 If these threads were merely thinner strands of the same material as 

 the very much thicker chromosomes, it is hard to understand why we 

 do not find all gradations in thickness between the chromosomes and 

 transverse threads. 



Heidenhain (1911) considers the linin to be the contractile substance 

 by which the movements undergone by the chromatin in prophase and 

 telophase are brought about. It is probably closely similar to cytoplasm 

 in nature, though plainly of a firmer consistency. 



(3) Nuclear Membrane and Karyolymph 



The mode of formation and nature of the nuclear membrane is 

 uncertain. It is possible that it is formed out of the linin framework of 

 the nucleus, or, on the other hand, it may be a condensation or precipita- 

 tion of the cytoplasm where it comes into contact with the karyolymph 

 or nuclear sap which accumulates between and within the chromosomes 

 at telophase. The telophase nucleus may in this case be conceived of as 

 lying in a vacuole full of karyolymph, the cytoplasm round the circum- 

 ference, and therefore in contact with the nuclear sap, becoming hardened 

 to form the nuclear membrane. Whatever its mode of origin, the 

 membrane becomes an integral part of the nucleus. 



As a rule, the nuclear membrane disappears in the late prophase, but 

 it may persist throughout the whole mitosis (e.g. many insects) or may 

 disappear in early prophase (e.g., Mesostoma, von Voss, 1914). 



(4) Karyosomes and Prochromosomes 



The term karyosome may be applied to any mass of chromatin large 

 enough to stand out conspicuously from the general nuclear groundwork. 

 Thus in a finely reticulated nucleus a comparatively small aggregation of 

 chromatin may be alluded to under that name, while a nucleus with a 



