250 THE PROTOZOA 



which is ordinarily thick and resisting disappears during division, and 

 the edges which are left gradually fade away into the cytoplasmic 

 reticulum, where they cannot be distinguished from the outer network. 

 In this case apparently, and possibly in other Protozoa, the membranes 

 arise from the substance of the cytoplasmic reticulum. Another 

 interpretation, however, is possible. Thus in some Coccidiida 

 {Klossia), Labbe ('96) and Siedlecki ('98) described the membrane as 

 very delicate and staining in the same manner as the chromatin net- 

 work, and Siedlecki adds : "In fact, it represents only a more condensed 

 part of this network." 1 Hertwig ('98) held a similar view of the 

 membrane in the case of Actinosphcerium, regarding which he says : 

 " In well-preserved specimens the nuclear reticulum and membrane 

 stand in very close connection, and frequently the two cannot be dis- 

 tinguished." 2 The best evidence, however, of the nuclear origin of the 

 membrane is given by Schaudinn ('94) in the developing nuclei of Cal- 

 cituba polymorpha, a rhizopod belonging to the family Miliolida? (Fig. 

 134, A). Here the young nucleus appears as a solid homogeneous 

 sphere of chromatin which gathers fluid from the surrounding plasm 

 and forms peripheral vacuoles. These vacuoles then pass to the inside, 

 and soon the entire nucleus appears vacuolated, a mere network of 

 chromatin. The chromatin then segregates into a central mass con- 

 nected by fibres with a peripheral layer of chromatin which forms the 

 nuclear membrane. The further history consists of the separation of 

 bits of the central mass, which pass along the radial lines to the mem- 

 brane, where they ultimately form a layer of chromatin similar to the 

 original. By the bursting of the membrane these are liberated and 

 recommence the cycle. 



In other forms, notably in Sporozoa, the nuclear membrane is 

 apparently of minor importance in the cell. Wolters ('91) described 

 a delicate membrane in some forms of Monocystis {e.g. M. agilis)\ 

 other forms, as Monocystis ascidice, have a thick membrane com- 

 posed of deeply staining fibrils, 3 while others of the same genus 

 have none at all. Various other forms of Protozoa also have 

 no nuclear membranes, the chromatin in such cases being distributed 

 throughout the cell. Examples of this type of nucleus are found in 

 all classes; among the Sarcodina, Gruber ('84) and Frenzel ('91), 

 among the Mastigophora, Butschli ('96) and Calkins ('98), among 

 the Ciliata, Balbiani ('60), Gruber ('84), and Bergh ('89) have de- 

 scribed them. In these various descriptions it is not always made 

 clear whether the distributed chromatin is merely a diffused nucleus 

 or whether each of the parts is not a single small nucleus which 

 divides by itself, as in the dividing granules described by Schewiakoff 



1 Siedlecki ('98), p. 806. - Loc. cit., p. 635. s Cf. Siedlecki, '99. 



