ii8 



PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 



plasm ; these discharge into the vacuole, which then bursts 

 to the exterior. 



Life-history.— Growth is followed by obliquely transverse division 

 into two (Fig. 50, D.L.). One half includes the "mouth," the other 

 has to make one. As well as this simple fission, a process of transient 



Pjq t^i. — Conjugation of Paramcecium aurelia- 

 stages. — After Maupas. 



-four 



Shows macronucleus (A^.) and two micronuclei (m.) in each 



of the two conjugates. 

 Shows breaking up of macronucleus, and multiplication of 



micronuclei to eight. 

 Shows the fertilisation in progress ; the macronucleus is 



vanishing. 

 Shows a single (fertilised) niicronucleus in each conjugate. 



conjugation also occurs. Two individuals approach one another closely, 

 the two nuclei of each break up, an exchange of pieces of the micro- 

 nucleus takes place ; the two then separate, each to reconstruct its two 

 nuclei (Fig. 51). This process is necessary for the continued health of 

 the species. 



Fig. 52. — Diagrammatic expression of process 

 of conjugation in Paramcecium aurelia. 

 — After Maupas. 



The two micronuclei enlarge. 



Each divides into two. 



Eight micronuclei are formed. 



Seven disappear ; one (darkened) divides into two. 



An interchange and fusion occurs, and the con- 

 jugates separate. 



The fertilised micronucleus divides into two. 



Each conjugate begins to divide, the micronucleus 

 of each half dividing into two, one of which 

 becomes the macronucleus, while the others form 

 the two normal micronuclei. The top line repre- 

 sents four individuals, each with a macronucleus 

 and two micronuclei. 



The details of the conjugating process have been worked out with 

 great care by Maupas and others. They differ slightly in different 

 species ; what occurs in P. aurelia is summarised diagrammatically 

 in Fig. 52. 



The micronuclear elements are represented by two minute bodies. 

 As conjugation begins, these separate themselves from the macronucleus. 

 The macronucleus degenerates, and each micronucleus increases in 



