STRUCTURE. 109 



no reason why he or any other evolutionist should doubt 

 that they have in them the indefinable rudiments of 

 thought. 



Structure. — The Protozoa are sometimes called " struc- 

 tureless," but they are only so relatively. For though they 

 have not stomachs, hearts, and kidneys, as Ehrenberg 

 supposed, they are not like drops of white of egg. Ou'r 

 eyes, when aided by the microscope, can distinguish struc- 

 ture in these simplest animals. They are simple as an egg 

 is simple when compared with a bird. 



The cell substance consists of a living network or foam, 

 in the meshes or vacuoles of which there is looser material. 

 Included with the latter are granules, some of which are 

 food fragments in process of digestion, or waste products in 

 process of excretion. 



The cell substance includes one or more nuclei, special- 

 ised areas which are essential to the life and multiplication 

 of the unit. In the Protozoa there are several conditions 

 under which the nucleus may exist. 



(1) In some adult forms, and in many spores or young forms, no 

 nucleus has yet been discovered. It is, however, unnecessary to preserve 

 the term " Monera " for such simple forms, as it is probable that nuclear 

 material does exi.st in some form even in these cases. 



(2) In some of the Ciliata the nucleus is diffuse, that is, it exists in the 

 form of a powder scattered through the medullary protoplasm, and is 

 only discernible after death by means of careful staining. In Opalin- 

 opsis the fine powder sometimes coalesces into a single nucleus. 



(3) In the majority of cases, notably in the Gregarines. the nucleus 

 is single, often large, and placed centrally ; from a consideration of the 

 cells of Metazoa we may call this the typical case. 



(4) In many of the Ciliata, e.g:, Parama'ciwn, the nucleus is double. 

 There is a large oblong nucleus and beside it a smaller spherical one. 



(5) In Opalina, from the intestine of the frog, and a few other forms, 

 there are very numerous nuclei, arranged in a .symmetrical manner in 

 the cell substance. In some cases these isolated nuclei have been 

 observed to unite to form one large nucleus just before binary fission 

 takes place. Of these various cases the diffuse condition is apparently 

 very primitive. 



The nucleus, when stained and examined under high powers, is 

 observed to be complex in structure. It consists of a nuclear network, 

 or a coil of chromatin threads. In the division of many Protozoa, as in 

 the cells of higher animals it plays an important part. During division 

 it passes from the resting to the active condition. The nuclear threads 

 or " chromatin filaments " loosen themselves from their coiled state, 

 and arrange themselves in a star at the equator of the cell, whence they 



