421 Mr. Lawson Tait on the 



nuclei are stained ; and when they are brought into focus under a quarter 

 objective, it requires only about the thirtieth part of a revolution of a 

 fine screw adjustment to bring the subjacent fusiform nuclei into clear 

 definition, that screw having 56 threads to the inch. The thickness of 

 the layer is therefore certainly not more than j~ of an inch. 



For the silver treatment I have emploj^ed a solution of lactate of silver 

 with the addition of some free lactic acid, and I have found the results 

 more definite than those given by the employment of a solution of the 

 nitrate. The method is advised by Serge Alferow, of Charkow. This 

 treatment displays an irregularity of size and arrangement of the cells 

 such as I do not see in those of any other epithelial surface, and which 

 I think must be due to the singleness of the layer. It is not due to my 

 method of treatment, for I have seen it in small pieces which I have 

 snipped off by scissors, without freezing on glass. The intercellular sub- 

 stance takes on the characteristic brown colour (Plate 11. fig. 3), and be- 

 tween certain cells slight ganglioniform enlargements of this colour are to 

 be seen in great numbers — the stomata spuria of Klein. I am in as great 

 doubt as that author whether these dark spots have any thing to do with 

 the subjacent canal-system. They may have ; but I have seen nothing as 

 yet which convinces me that they are other than mere extensions of the 

 peculiar intercellular substance which first takes up the argentic stain. 

 That this stain is more than a mere filling up by the darkened solution 

 of intercellular gutters must be the conviction of any one who works to 

 any large extent with silver fluids ; for in other tissues it may be seen 

 to affect nuclei with avidity, as notably, in my own experience, it does in 

 the placenta and in the ovary. Certain groups of cells in epithelial 

 surfaces are seen to take up this colour without apparent meaning ; but 

 this is much less frequent in the cord than in the amnion, &c. 



The groups of small darkly coloured cells or nuclei which are now 

 admitted to mark the stomata of serous membranes are numerous on 

 the cord ; but I am bound to state that my experience of them had to be 

 extended to other membranes before I could admit for them the inter- 

 pretation which they had in the opinion of others, and notably of Koster. 

 They are unquestionably the orifices of the vast system of canals which 

 forms the basis of the cord and upon which it solely depends for its 

 nutrition. Round their orifices, or within their entrance (I have not yet 

 decided which), are to be seen these small darkly shaded cells. I doubt 

 if they are endothelia. I believe that rather they are young epithelia in 

 growth ; that, in fact, these stomata are the points of growth and ex- 

 tension of the epithelial layer of the cord, and that the groups of 

 darkly coloured cells of larger size to which I have already referred are 

 the same in further progress. It certainly is the fact that round the 

 stomata the uncoloured cells are often of much smaller size than they are 

 at a little distance away. At a few points I have been fortunate 

 enough to see these stomata leading directly into a canal, into which the 



