176 THE president's address. 



its most elaborate type of structure the chromatin -grains are 

 suspended at the nodal points of a delicate framewoi-k, which 

 contains a fluid nuclear sap in its meshes, the whole enclosed 

 in a distinct membrane. Other bodies, such as nucleoli, distinct 

 in nature from chromatin, and kinetic mechanisms, so-called 

 centrosomes or centrioles, may be present in addition. All these 

 structural elements, framework, nuclear sap, membrane, nucleoli 

 and centrioles, may be regarded as so many acquisitions in 

 the course of evolution, ancillary to the original chromatin, the 

 substance of primary imjwrtance in the life of the organism. 



With differentiation of nucleus and cytoplasm the organism 

 becomes what is commonly termed a cell ; I shall therefore denote 

 this type of structure briefly the cellular grade. A cell may 

 be defined as a lump or corpuscle of protoplasm, differentiated 

 into cytoplasm containing at least one nucleus. It is not 

 correct, in my opinion, to speak of Bacteria as cells ; they are 

 Protista of a lower grade, in which the type of structure proper 

 to a true cell has not beeii attained, in which concentration of 

 the chromatin to form a nucleus has not taken place, and in 

 which, consequently, no distinct cytoplasm has been differentiated. 



The difference of structure between these two types or grades 

 may seem at first merely one of trivial detail. There are some 

 considerations which tend to show that it is far from being so, 

 but that, on the contrary, the attainment of the cellular grade of 

 structure was attended with the most momentous consequences. 



In the first place, let me draw your attention to certain re- 

 markable phenomena of life about which I have said nothing 

 so far. In all forms of the higher visible world of living things 

 we find universally, in animals and plants alike, the existence 

 of sex and sexual differentiation. The essential feature of the 

 sexual process, throughout the whole series, is the production by 

 each individual, male or female, of peculiar cells, termed gametes, 

 which are set free from the body, or at least from the organs in 

 which they arise. The gametes produced by each sex are very 

 different in size, form and appearance. Their destiny is for a 

 gamete of one sex to unite with one of the other, and then their 

 bodies, and especially their nuclei, fuse completely to form a 

 single cell with one nucleus ; this constitutes the sexual act, 

 for which various special words are in general use, such as con- 

 jugation, fertilisation, etc., but for which we may employ the 



