ANIMAL CELLS. 3 



nised (by Flemming and others) in many cells, and are probably of 

 universal occurrence. They are very often double, the twin spheres 



FIG. 2. A CELL (WHITE BLOOD COK- 

 PUSCLE) SHOWING ITS ATTRACTION - 



SPHEEE. 



In this, as in most cases, the attraction- 

 sphere lies near the nucleus. 



FIG. 3. OVUM OF ASCARIS, SHOWING 

 A TWIN ATTRACTION-SPHERE. (v. 

 Beneden.) 



The nucleus with its contorted filament 

 of chromoplasm is represented, but the 

 protoplasm of the cell is not filled in. 



being connected by a spindle-shaped system of delicate fibrils (achro- 

 matic spindle) : this duplication invariably precedes the division of a cell 

 into two (fig. 3). 



A cell-membrane is rarely distinct in animal cells, nor has its chemical 

 nature been sufficiently investigated. It is formed by the external 

 layer of the protoplasm. 



The nucleus of the cell (fig. 1, n) is a minute vesicle, spherical, 

 ovoidal or elongated in shape, embedded in the protoplasm. It is 

 bounded by a membrane which incloses a clear substance (nuclear 

 matrix), and the whole of this substance is generally pervaded by an 

 irregular network of fibres, some coarser, others finer (intranuclear net- 

 worlc). This intranuclear network often exhibits one or more enlarge- 

 ments, which are known as the nudeoli. The nuclear membrane, 

 intranuclear fibres and nucleoli all stain deeply with haematoxylin and 

 with most other dyes; this property distinguishes them from the 

 nuclear matrix, and they are accordingly spoken of as chromatic, com- 

 posed of chromoplasm, the matrix as achromatic. Sometimes instead of 

 uniting into a network the intranuclear fibres take the form of con- 

 voluted filaments, having a skein-like appearance. This is always the 

 case when a nucleus is about to divide, but it may also occur in the 

 resting condition. These filaments may sometimes be seen with very 

 high magnifying powers to be made up of fine juxtaposed particles 

 arranged either in single or multiple rows; thus imparting a cross- 

 striated appearance to the filament (see fig. 4, B, c). The fibres within 

 the nucleus have been observed to undergo spontaneous changes of 

 form and arrangement, but these become much more evident during 

 its division. The division of the protoplasm is always preceded by 



