io6 PROTOZOA THE SIMPLEST ANIMALS. 



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

 divide into two groups, which retreat from one another, and become the 

 daughter nuclei of two daughter cells. In short, karyokinesis has been 

 observed here as elsewhere (see p. 46). 



While we cannot at present define the physiological import of the 

 nucleus, we must recognise its importance. Thus Bruno Hofer has 

 shown that when an Amceba is cut in two, the part with the nucleus 

 lives and grows normally, while the part without any nucleus sooner or 

 later dies ; and Balbiani has observed that in the case of Infusorians cut 

 into pieces, those parts which have nuclei survive, while if no nucleus is 

 present in the fragment, the wound may remain unhealed, and death 

 ensues. There seems no reason why one may not combine the view of 

 Weismann, that the nucleus bears the essential hereditary substances, 

 with the view that it is a trophic, or, at any rate, a vital centre in the 

 cell. 



In naked Protozoa the outer part of the cell-substance 

 ("ectoplasm") is often clearer and less granular than the 

 inner part ("endoplasm "), but this difference is a physical 

 one of little importance. In corticate Protozoa there is a 

 more definite rind or thickened margin of cell-substance. 

 Outside this there may be a " cuticle " distinct from the 

 living matter, sometimes consisting of chitin, or gelatin, or 

 rarely of cellulose. The cuticle may form a cyst, which is 

 either a protection during drought, or a sheath within which 

 the unit proceeds to divide into numerous spores. More- 

 over, the cuticle may become the basis of a shell formed 

 from foreign particles, or made by the animal itself of lime, 

 flint, or "horny" material. 



In the cell-substance there may be bubbles of water taken 

 in with food particles (food vacuoles), contractile vacuoles, 

 fibres which seem to be specially contractile (in Gregarines), 

 spicules of flint or threads of horn-like material, which may 

 build up a connected framework, and the pigments already 

 mentioned. 



Reproduction of Protozoa. 



Growth and reproduction are on a different plane from 

 the other functions. Growth occurs when income exceeds 

 expenditure, and when constructive or anabolic processes 

 are in the ascendant. Reproduction occurs at the limit of 

 growth, or sometimes in disadvantageous conditions, when 

 disruptive or katabolic processes gain some relative pre- 

 dominance. 



