154 THE PROTOZOA 



pronuclei is to be regarded as the equivalent of the division of 

 the synkaryon which took place ancestrally after syngamy. While, 

 however, there is a general agreement that partial karyogamy 

 (conjugation) is to be derived from total karyogamy (copulation), 

 it is very doubtful if the two conjugants in Infusoria represent 

 simple gametes ; it is more probable that the type of syngamy 

 characteristic of Infusoria is derived from an ancestral condition 

 in which each conjugant produced a number of minute gametes 

 (swarm-spores) which copulated (compare especially Popoff, 125, 

 and Hartmann, 116, and see p. 453, infra). On this view the 

 divisions of the micronucleus represent a primitively much larger 

 number of divisions which produced the numerous gametes, and 

 the conjugants themselves are not to be regarded as true gametes, 

 but rather as gametocytes or gamonts. 



Having now illustrated by typical examples the various forms 

 which the syngamic process takes in Protozoa, we may conclude 

 this chapter by a consideration, necessarily brief, of the problem 

 of the significance and origin of syngamy and sex. This is a 

 problem which has a vast literature, and it is only possible here to 

 indicate in outline some of the theories that have been put forward, 

 none of which can claim to be a complete solution of one of the 

 profoundest mysteries of the living substance and its activities. 



Considering first the fertilization of the Metazoa, it is evident 

 that the union of the spermatozoon with the ovum has two prin- 

 cipal results. In the first place the spermatozoon brings with it 

 a pronucleus, the equivalent of that contained in the ovum, but 

 derived from a distinct individual, and therefore possessing different 

 hereditary tendencies acquired from its own particular ancestral 

 history. The union of the male and female pronuclei brings about, 

 therefore, a process for which Weismann has coined the term amphi- 

 mixis that is to say, a mingling of different hereditary tendencies 

 in one and the same individual. In the second place the spermato- 

 zoon produces a result which may be termed briefly " developmental 

 stimulus " (Entwicklungserregung) that is to say, it produces 

 a disturbance in the equilibrium of the protoplasmic body of the 

 ovum which causes it to start on a course of cell-division oft-re- 

 peated, a process of cleavage which converts the unicellular ovum 

 into the mass of cells which supplies the material for the building 

 up of the multicellular body. It is very probable that the develop- 

 mental stimulus is supplied by the greatly-developed centrosome 

 of the spermatozoon, that of the ovum having completely atrophied, 

 apparently, after the completion of its maturative processes. 



The introduction of a male pronucleus that is to say, the process 

 of amphimixis can be effected only by the spermatozoon. But 

 the researches of Loeb and others have demonstrated fully that the 



