250 Papers from the Department of Marine Biology. 
terns needed explanation and some teased fresh cells were put between 
cover-glass and slide and Flemming and other fixatives were run in 
under a 2 mm. apochromatic immersion lens with the best condition 
of lighting. 
According to many investigators (Kite (22), Chambers, etc.), the 
achromatic contents of the nucleus during life are a thin colloidal sol 
which sometimes approaches a delicate gel in consistency. In the 
present case the writer has rather hurriedly dissected the nucleus under 
the Barbour apparatus and noted that it is a viscous gel, which is very 
soft and almost a sol in its fluidity. No traces could be observed of any 
fibrillation, either by an optical examination or by a physical examina- 
tion with the glass needles. 
This viscous fluid was carefully watched as the fixing fluid came in 
contact with it, and the following process seemed to take place: The 
fixative (Flemming’s fluid seemed to give the best picture) advanced 
slowly on a more or less straight front with a fairly definite line between 
the portion that it had invaded and the parts as yet untouched. It 
seems probable that this fluid with its dense content of salts advances 
by a combination of an osmotic action and a mechanical rupture of the 
fixed wall that is always in front of it. The line of division mentioned 
above was seen to bend into a series of rounded pockets, which were 
broken inward by the increasing osmotic pressure and convection 
currents of the fixative burst into the unfixed mass of nuclear material. 
They fixed the new surfaces thus brought into contact with them, and 
then the process of bending inward portions of the new line of contact by 
osmotic pressure and again breaking through to form new cavities was 
resumed. As the fixative penetrates further, it becomes diluted and 
less vigorous in its action, and the united osmotic action of its salts 
on the line of contact becomes less. Consequently a larger portion of 
the line is bent in and broken, the successive pockets of inruption 
become greater, and the resulting mesh becomes larger, as seen in the 
fixed specimen. 
In fixation by gases, such as by formol and by osmic-acid vapor, the 
penetration is by diffusion and the mechanical factor does not enter 
into the process. Also, in fixation by many fluids the diffusion of 
gases proceeds ahead of the osmotic-pressure action and we have 
nuclei whose contents are apparently homogeneous. 
It can be seen that this combined chemical and mechanical action 
would account for the coagula that have been described for many body 
fluids and which are so characteristic for the blood and lymph fluids 
of many animals. An examination of such a coagulum is sufficient in 
many cases for an expert histologist or pathologist to state what fluid 
it is, what fixation has been used, and how far the cell or tissue ex- 
amined was from the surface of the bit of tissue that was fixed. 
One further experiment was tried, to throw light on the point of issue 
between Magini and Coggi as to the meaning of the dorsal meniscus 
