142 



PRACTICAL HISTOLOGY. 



[VII. 



Protoplasm 



Nucle 



Xuclc ar Membrane. 

 Xuclear Network. 



The nucleus (fig. 105), bounded by a nuclear membrane, com- 

 posed of two layers, an outer one, which does not stain (achromatic), 

 and an inner one, which does (chromatic). "Within the membrane 



is an intranuclear network or 

 karyomiton (Kapvov, a kernel) 

 or karyomitoma, consisting of 

 a reticulum of threads or fine 

 fibres, arranged sometimes in 

 the form of a regular network. 

 As these threads, or at least 

 particles in them, stain readily 

 with certain dyes, e.g., safranin, 

 FIG. 105. Connective-Tissue Corpuscle from they have been called chromatiii 



or composed of chromoplasm. 



In the meshes of this more or less perfect network lies the nuclear 

 fluid, which, however, does not stain with certain pigments : it has 

 been called achromatin. 



In the meshes of the reticulum lies one usually more than 

 one -nucleolus. It is more refractile than the rest of the nucleus. 

 Generally, however, as stated, two or more nucleoli are present, 

 and they seem to differ in their chemical constitution, so that 

 Flemming speaks of principal and accessory nucleoli. Many of 

 the bodies described as nucleoli are really parts of the intranuclear 

 fibrillar network seen in optical section. Other observers have 

 applied different terms to these structures, but here it is not 

 necessary to multiply terms. The "attraction sphere" existing in 

 the protoplasm of some cells seems to exercise some influence on 

 the dividing nucleus. 



The great majority of cells reproduce themselves by indirect 

 cell-division or mitosis, and in this process the network within 

 the nucleus plays a most remarkable part. The division of a cell 

 is always preceded by the division of the nucleus. Starting from 

 the resting nucleus, where the threads are not well developed, soon 

 two poles appear in the nucleus, and then the threads grow 

 thicker, more numerous and tortuous, forming the convolution 

 stage. The various stages are indicated in the following 

 scheme : 



Mitosis. 



1. Resting nucleus. 



2. Skein or spirem. 



the 



The mother nucleus showing the fibrils 

 reticulum or network stage. 



Close skeiu of fine convoluted fibrils, then thicker 

 loops running from polar to antipolar regions, the 

 nucleoli disappear. 



3. Cleavage of fibrils. Each loop (usually v- sna l )(1( l) splits longitudinally 

 into two and the achromatic spindle appears. 



