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 



