72 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



former case all the functions are performed by the same cell, while in 

 the latter there are special reproductive cells. The permanent conjugation 

 of Vorticella, however, where a small active microzoid unites with a larger 

 more passive macrozoid, appears to be equivalent to an act of sexual repro- 

 duction i.e., to the union of a male cell (sperm) with a female one (ovum). 

 The process in Vorticella is in reality a very complicated one, and the 

 paranuclei (micronuclei) are alone concerned in it. The ordinary nuclei 

 (macronuclei) appear to have as their function the regulation of nutrition. 

 In many free-swimming ciliated Protozoa "temporary" conjugation takes 

 place between two equal-sized individuals. They apply themselves closely 

 by their oral surfaces and exchange parts of their paranuclei, after which 

 separation takes place, the nuclear apparatus is reconstructed, and fission 

 is resumed. It has been demonstrated that, in ciliated Protozoa, this 

 process is necessary for the continuance of the species, as without it asexual 

 reproduction cannot take place beyond a certain number of generations. 

 In one of the simplest cases (Colpidium) the changes which take place in 

 the paranucleus are as follows: The division is always of the "indirect" 

 kind (mitosis, karyokinesis), and involves more than simple elongation, 

 constriction, and division; (1) Paranucleus divides into two, n 1 , n 1 , and 

 (2) these subdivide again n 2 , n 2 , n 2 , n 2 . (3) Of these four parts three are 

 absorbed by the protoplasm, while the fourth again divides into two, n 3 , n 3 . 

 (4) One of these migrates into the other individual and fuses with the cor- 

 responding part. (5) Each individual now contains a conjugation nucleus 

 [n 3 + n 3 ] formed by fusion of -|th of one paranucleus with ^th of the other. 

 This compound structure divides twice, two of the quarters passing to 

 one end and two to the other. Meanwhile the original nuclei have been 

 absorbed and the individuals have separated. Transverse fission now takes 

 place, the quarters above-mentioned becoming the nuclei and paranuclei 

 of the daughter forms. The complete process may be graphically repre- 

 sented as follows, where N = nucleus, n paranucleus, and the indices are 

 written above (e.g., n 3 ) for one individual and below (e.g., n 3 ) for the 

 other. Elements enclosed in parentheses are absorbed, v subdivisions 

 of conjugation nucleus. 



INDIVIDUAL 1. INDIVIDUAL 2. 



c 



H v "- i ^2 4 ". n s 1 ' no 



n s / 



(N) 







+ n s ]^- 



I 



(N 



new 



N and n = 

 of 



new = 



fvM v 



Nandn 



The processes of conjugation just described involve complete reconstruc- 

 tion of the nuclear elements by (a) rejection and absorption of parts of the 



