66 BIOLOGICAL LECTURES. 



of molecules always form a closed surface and a molecular 

 envelope for the organism. 



Since the molecules within a semi-fluid mass also move freely 

 over one another there is friction developed between them. 

 This friction is known as viscosity, and differs in fluids of 

 differing densities and chemical compositions, just as, in fact, 

 the surface-tension of a unit- surf ace, formed of different fluids, 

 differs, owing to the differing dimensions and properties of the 

 superficial layer of molecules of each fluid. The specific 

 viscosities and surface-tensions manifested by any given fluid 

 are therefore correlated, so that we may infer that these two 

 properties of fluent living matter are also correlated. Since, 

 again, living matter, as found in the cell, is not a homogeneous 

 body or compound, it will be plain that the correlative disturb- 

 ances in viscosity and surface-tension, due to the processes of 

 metabolism and the associated interplay of osmotic changes, 

 are so complex, when considered together with still other facts, 

 that it will be difficult, if not impossible, in the present state 

 of our knowledge, to trace all the steps of their interrelations 

 and interdependences in any given case. 



The recent progress in the study of the process of fertiliza- 

 tion or conjugation shows that dynamical considerations must 

 here also be taken into account. As is well known, the nucleus 

 and archoplasm undergo alternate expansion and contraction 

 in linear dimensions. The male and female pronuclei increase 

 greatly in size during the phases just before conjugation. In the 

 egg of Ascaris, for example, I have noticed that the pronuclei 

 soon assume a globular form and rapidly grow into large 

 spherical bodies by absorbing substance from the surrounding 

 cytoplasm. This rapid growth and spherical form show that 

 surface-tension is being disturbed uniformly over the whole 

 surface of the nucleus, otherwise its form could not remain 

 spherical. The remarkable growth of the rays of the asters 

 has the same meaning and must also be interpreted, in part, as 

 a physical process involving radial interdiffusion of heteroge- 

 neous molecules to and from the centrosomes into the nucleus 

 and cytoplasm, consequent osmotic disturbances, metabolism 

 and changes of surface-tension. The comparison of the astral 



