264 CONCLUSION [CH. 



to imagine the evolution of the more primitive forms of 

 Protozoan cell, and by the aggregation or multiplication of 

 the biococci to suppose that a simple nucleus could arise. 



Evidently, as MINCHIN says, the cell and nucleus of the 

 Metazoa are highly organised structures which must have 

 passed through many stages of evolution before they 

 reached their present form. To obtain clear indications of 

 the more primitive condition the comparative cytologist 

 will need to turn to the Protozoa, among which there is 

 probably much greater variety of cell and nuclear structure 

 than in all the Metazoa taken together. Even in the Proto- 

 zoa, however, many kinds of differentiation have taken 

 place in various directions, and the search for conditions 

 that may be regarded as primitive is no easy one. On all 

 grounds, however, it seems certain that just as the cytolo- 

 gist of the future whose interest is centred on function will 

 be carried by his studies to the borderland between biology 

 and physico-chemistry, so the student of cellular morpho- 

 logy will be driven to seek for the origins of cell-structure in 

 the Protozoa, Protophyta and Bacteria. 



Cytology in the last few years has been in a condition not 

 unlike that of Zoology in the third quarter of the nine- 

 teenth century. After the publication of the Origin of 

 Species morphological and systematic Zoology, carried 

 away by an illuminating and unifying idea, devoted itself 

 to the testing of the evolutionary theory and to seeking 

 fresh evidence in its support, and allowed itself to become 

 divorced from its natural partner, Physiology. So, since 

 the discovery of a possible relation between Mendelian 

 segregation and chromosome behaviour, and between spe- 

 cific chromosomes and sex-determination, descriptive cyto- 

 logy has been occupied very largely with the investigation 

 of these problems, and has allowed the closely allied experi- 



