vi CYTOLOGY 



special mention. With the Protista the case is different, but even these 

 forms have been treated in a very brief and eclectic manner. This is 

 partly because, like most cytologists, the author has unfortunately very 

 little first-hand knowledge of the cytology of this group, and partly 

 because Protistan cytology has not up to the present time helped us very 

 much to gain an insight into those aspects of the nucleus and its activities 

 with which we are here concerned. 



As the science of Physiology is almost entirely founded on the physio- 

 logy of the higher Vertebrates, so is Cytology mainly the cytology of the 

 Metazoa and Metaphyta. In both cases the greater simplicity of the 

 technique of dealing with the larger forms has no doubt been partly 

 responsible for this state of affairs, but there is another and more 

 important reason. The functions of organs and the meanings of pro- 

 cesses are far easier to interpret when the organs are complicated and 

 the processes specialized than when they are simple or generalized. The 

 specialization of the nerve cells of the Metazoa for conduction of im- 

 pulses, of their muscle cells for contraction, and of their gland cells for 

 secretion has enabled physiologists to gain a far deeper insight into the 

 physiology of these processes than they could have obtained from the 

 study of the Protistan cell in which no one function dominates over 

 the others. Similarly, the high specialization of the mitotic processes 

 in Metazoa and Metaphyta has afforded cytologists. an insight into their 

 meaning far greater than could ever have been obtained from the study 

 of the less specialized nuclei of most of the Protista. In order properly 

 to understand any process, however, it is necessary to know as many 

 variants of it as possible, as well as the steps by which it has been 

 evolved. For this reason a very brief account of some of the more 

 striking differences between the Metazoan and Protistan nuclei has 

 been given. 



Each branch of Science presents its own special technical difficulties 

 to its students. Cytology has at least its full share of these. The objects 

 with which the cytologist has to deal are extremely minute, and indeed 

 his analyses are almost invariably limited merely by the imperfection of 

 his optical instruments and technique. However powerful the former 

 and perfect the latter, he is sure that below the limits of visibility there 

 exists a morphological complexity at least as great as that which he has 

 already revealed. At present he has no methods comparable with those 

 of the chemist and physicist for dealing indirectly with ultramicroscopic 



