THE NUCLEUS 75 



nucleus consists of a large mass of chromatin suspended in the 

 nuclear sap. In some cases no other structural elements can be 

 made out ; in others the nuclear sap contains granules of peripheral 

 chromatin varying in size from the most minute and scarcely 

 visible particles to distinct grains. For a simple nucleus of this 

 type the term " protokaryon " has been proposed ; it is just such 

 a nucleus as may be imagined to have arisen by a concentration 

 of chromidiosomes at one spot in the cell-body, and in many cases 

 such nuclei can be seen to be formed actually in this manner. The 

 kinetonucleus of trypanosomes may be considered as a nucleus of 

 this type in which the single mass of dense chromatin fills almost 

 or quite completely the space in which it lies. In other cases there 

 may be a clump of chromatin-grains more or less equal in size, 

 filling the nuclear cavity, as in the nucleus of hamogregarines. 

 When there are numerous grains of chromatin, those placed super- 

 ficially may be united to form a limiting layer which may be termed 

 a "false" or " chromatinic " membrane, in distinction to a true 

 nuclear membrane, which is an achromatinic structure. Even in 

 nuclei of the most simple type, however, substances or structures 

 accessory to the chromatin are probably always present. 



In the first place, it is very probable that the grain or grains of 

 chromatin do not lie loosely and freely in the nuclear vacuole, but 

 are suspended in it, in all cases, by a delicate achrornatinic frame- 

 work, presenting the appearance of a fine network or reticulurn, at 

 the nodes of which the chromatin-grains are lodged. It is true that 

 in many of the minute and primitive forms no such framework has 

 been made out, and is believed by many observers to be absent ; 

 but on that view it is difficult to account for the definite position 

 of the chromatin, its changes of position during division, and the 

 frequent appearance, during this process, of an achromatinic spindle, 

 phenomena that may be noted even in the simplest cases. The 

 achrornatimc framework is often very fine and delicate, and its 

 substance stains feebly or not at all with the colouring matters 

 commonly employed in microscopical technique ; hence it is very 

 probable that it has often been overlooked in cases where it is 

 really present. When there is but a single mass of chromatin, or 

 one grain very much larger than all the others, the achromatinic 

 reticuluni presents the appearance of very delicate threads of 

 limn radiating from the principal mass of chromatin to the 

 periphery. When, on the other hand, there are numerous grains 

 more or less equal in size, the reticuluni is seen as fine lines passing 

 from each grain of chromatin to each of the grains adjacent to it. 

 In all probability the apparent " threads " of the reticulum are but 

 the optical expression of the walls or partitions separating alveoli, 

 and there is no reason for considering the achroniatinic reticulum or 



