REPRODUCTION OF PROTOZOA. 117 



either permanently or temporarily. This is an incipiently 

 sexual process ; it is the analogue of the fertilisation of an 

 ovum by a spermatozoon. In many cases, moreover, 

 there is a difference between the two conjugates, analogous 

 to the difference between ovum and spermatozoon. 



(1) It is one of the recurrent phases in the life history of some of the 

 simplest Protozoa (Proteomyxa and Mycetozoa) (see p. 107), that a 

 number of amoeboid units flow together into a composite mass, which 

 has been called a " plasmodium" 



(2) It is known that more than two individual Sporozoa and other 

 forms -occasionally unite. To this the term " multiple conjugation " 

 has been applied. 



(3) Commonest, however, is the union of two apparently similar 

 individuals, either permanently, so that the two fuse into one, or 

 temporarily, so that an exchange of material is effected. Permanent 

 conjugation has been observed in several Rhizopods, Infusorians, and 

 Sporozoa. Temporary conjugation is well known in not a few ciliated 

 Infusorians, and it is possible that a curious end-to-end union of certain 

 Sporozoa is of the same nature, or it may be of the nature of a 

 " plasmodium " formation. The formation of small spores (gametes) 

 which conjugate is not uncommon. 



(4) There are some cases where one of the conjugating individuals 

 is larger and less active than the other. Thus in Vorticella, a small 

 free-swimming form unites and fuses completely with a stalked indivi- 

 dual of normal size. This "dimorphic conjugation" is evidently 

 analogous to the fertilisation of a passive ovum by an active sper- 

 matozoon. In Volvox this is even more obvious, for the small and 

 active cells, both in shape and method of formation, recall the sper- 

 matozoa of higher forms. 



Significance of Conjugation. The precise interpretation of 

 conjugation is uncertain. We may regard it as a mutual rejuvenescence, 

 each unit supplying some substances or qualities which the other lacks ; 

 or we may regard it rather as a process by which the average character 

 of the species is sustained, peculiarities or pathological variations of one 

 individual being counteracted by other characters in the neighbour 

 (apparently no near relation) with which it conjugates ; or we may see 

 in it a source of variation as the result of new combinations among 

 the essential hereditary substances. The researches of M. Maupas have 

 thrown much light on the facts, and some of his results deserve summary. 



It has been often alleged that the subsequent dividing is accelerated 

 by conjugation ; but Maupas finds that this is by no means the case. 

 The reverse in fact is true. While a pair of Infusorians (Onychodromus 

 grandts] were engaged in conjugation, a single individual had, by 

 ordinary asexual division, given rise to a family of from forty thousand 

 to fifty thousand individuals. Moreover, the intense internal changes 

 preparatory to conjugation, and the general inertia during subsequent 

 reconstruction, not only involve loss of time, but expose the Infusorians 

 to great risk. Conjugation seems to involve danger and death rather 

 than to conduce to multiplication and birth. 



