CHAPTER II 



METAZOA— INTRODUCTORY REMARKS 



In the phylum Protozoa we had to do with single cells living inde- 

 pendently. The individual cell multiplies from time to time, by fission 

 or otherwise, but the cells so arising separate and lead an independent 

 existence like the parent. In all the members of the animal kingdom 

 outside the Protozoa — commonly grouped together under the name 

 Metazoa — ^the life-history commences with a stage in which the indivi- 

 dual is a single cell — a zygote — which as in the case of the Protozoa 

 multiplies by a process of fission repeated over and over again, but in 

 this case the successive generations of cells produced by the process of 

 fission do not break apart and lead an independent existence. On the 

 contrary they remain as a coherent mass of cells which, in correlation with 

 the repeated fission of its component cells, exhibits growth in size. Just 

 as in the case of the Protozoa, the process of fission slackens ofi in due 

 course, so that the cell-mass does not increase in size indefinitely but 

 merely attains to a more or less definite full-grown size. This mass of 

 cells forms the body of the Metazoon, an individual of a higher order 

 than the cell-individual seen in the Protozoa, for it is composed of a mass 

 of cells which cohere together and have their individualities merged in 

 that of the whole. 



In a typical simple Protozoon the cell-individuals are unspecialized ; 

 each is just like its forebears. In the body of the Metazoon on the 

 other hand the successive generations of cells which come into existence 

 during the building up of the fully developed body become more and 

 more specialized : they gradually lose the primitive unspecialized 

 character of their zygote ancestor and, with the loss of the unspecialized 

 ancestral character, they lose for the most part their capacity for con- 

 jugating in a process of syngamy with other cell-individuals. At one 

 or more points in the body however there remain nests of cells which do 

 not become side-tracked on any path of specialization for particular 



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