128 



THE PROTOZOA 



of cytoplasm. Such unions are distinguished as plastogamy (or plasmogamy) 

 from true syngamy. Plastogamic union may be temporary or permanent ; 

 in the latter case it leads to the formation of plasmodia, as in the Mycetozoa 

 (p. 239). The significance of plastogamy is obscure in many cases, but in 

 some it may perhaps be comparable to the association of gametes already 

 described, and in this way may throw light on some cases of so-called 

 " autogamy" (see p. 138, infra). 



A further case of unions which are not in any way sexual in nature is seen 

 in the remarkable phenomena of agglomeration exhibited by some Protozoa 

 for example, trypanosomes. In this case the organisms adhere to each other 

 by the posterior or aflagellar end of the body, apparently by means of a sticky 

 secretion formed by the kinetonucleus, so that large clumps are formed 

 composed of numerous individuals. The phenomena of agglomeration are 

 associated with conditions unfavourable to the parasite, and appear to be 

 due to the formation of special substances, agglutinins, in the blood of the 

 host. Similar phenomena are well known in bacteria as agglutination, since 

 in this case the agglutinated individuals are unable to separate, while in 



FIG. 70. Precocious association and neogamy of gametocytes in gregarines. 

 A, Diplocystis minor, parasite of the cricket: m., common membrane uniting 

 the two associates ; g., grains of albuminoid reserve-material. B, Cystobia 

 holothurice, parasite of Holothuria tubulosa, showing the two nuclei in an 

 undivided body. A after Cuenot, magnified about 120 diameters ; B after 

 Minchin. 



the case of trypanosomes that are agglomerated it is possible for the indi- 

 viduals to become free again if the conditions are ameliorated. In other 

 Protozoa, also, phenomena of the nature of agglomeration are seen in de- 

 generating forms (see p. 209, infra). 



Certain aspects of syngamy must now be discussed in more 

 detail namely, the relation of syngamy to the life-history as a 

 whole ; its occurrence in the world of living beings ; its significance 

 for the life-cycle ; and its effects on the species and the individual. 



1. Syngamy in Relation to the Life-History of the Organism. 

 In any living organism the principal manifestation of vital activity 

 is the power of assimilation, resulting in growth. As a general 

 rule, however, the growth of an organism is not indefinite, but has 

 a specific limit ; an individual of a given species does not exceed 

 a certain size, which may be variable to a slight extent, but which 

 is fairly constant for normal individuals of the species in question 

 under similar environmental conditions. When the limit is 



