200 



GENERAL PHYSIOLOGY 



co?yicsdes, and their subsequent extrusion (Fig. 75, p. 196). Fer- 

 tilisation, therefore, consists in the union of a mature egg-cell with 

 a sperm-cell, in which process the latter seeks the former by its 

 own locomotion. We shall become acquainted with the mode of 

 locomotion later in considering the phenomena of movement. 



The process of the union of two cells is a phenomenon that 

 occurs not only in sexual reproduction but is constantly met with 

 among unicellular organisms, where sexual differentiation cannot 

 be said to exist. There, in the Protista, it is known by the name 

 of conjugation. Conjugation occurs even among the unicellular 

 shell-bearing Bhizopoda, e.g., in Bifflugia, which is provided with 

 a delicate capsule. In this genus two, and sometimes three, four, 







f 



\h. 



Fig. 81. — Various forms of spermatozoa, a, From a bat {1 espeitcgo noctuma) (after 

 Ballowitz) ; b and c, from the frog ; d, from the finch ; e, from the sheep ; / and 

 g, from the pig. (After Schweigger-Seidel.) h, From a medusa ; i, from a monkey 

 (Cercopithecus) ; I, from a crustacean. (After Glaus.) k, From the round- worm (after 

 Boveri). 



or even more, of the sluggish protoplasmic forms creep closely 

 together; their protoplasmic bodies lie in contact with one 

 another, then coalesce into a common mass, and finally separate 

 after the protoplasm of the various bodies has mixed and certain 

 changes in the nuclei have taken place.^ The phenomena of 

 conjugation in ciliate Infusoria have been studied very thoroughly 

 by Blitschli (76), Balbiani ('61), Maupas ('88), A. Gruber ('86, 2), 

 and R. Hertwig ('88 — '89). Parammcium is an oblong infusorian, 

 completely ciliated upon the outside, and constitutes an 

 extraordinarily favourable object for cell-physiological investiga- 

 tions of the greatest variety. Paramwcia, visible to the naked 

 eye, may be cultivated in great quantity in decomposing hay- 

 infusions and may be kept in stock. It is frequently observed 

 that an epidemic of conjugation suddenly appears throughout the 

 whole culture, so that almost none but conjugating individuals are 

 found. The phenomena of conjugation are as follows: — Two indi- 

 1 Gf. Verworn ('90, 1). 



