MINUTE STEUCTUEE OF THE SPEEMATOZOON OP NEWT. 337 



agents. It stains more readily than the body and membrane, 

 but not so deeply as what I have termed the neck. Towards 

 the extreme end, from tapering very regularly, the head 

 becomes somewhat abruptly more constricted for the last few 



• Fig. 1. 



micromillimeters of its length, and is here, in unstained pre- 

 parations, more highly refracting than the rest, its substance 

 appears more dense ; probably this end portion is solid and 

 the remainder hollow (of which preparations stained with car- 

 mine present very much the appearance), and shows a double 

 contour. At the extreme point of this head there is a minute 

 barb (see woodcut, fig. 1) . In successful preparations it may be 

 very distinctly seen and readily measured, and this even when 

 unstained. I have already-^ referred to its existence, and on 

 further examination of better preparations do not find it so ultra 

 minute as I at first thought it. It is indeed of very appreciable 

 magnitude, being in breadtli about 1'5 mic. m. (0*0015 mm,), and 

 in length 2*0 mic. m. (0'002 mm.), though obviously accurate 

 measurements of such objects are difficult, for to detect the 

 actual termination of an impalpably fine point is not always 

 possible.^ 



Such a determinate and remarkable structure as that here 



' See this Journal ante, vol. Ixxxii, N. S., 1882. 



^ In such measurements I have found great advantage in the use of a cob- 

 web micrometer, admirably constructed by Messrs. Ross, which has the second 

 web, which is usually fixed, movable ; this both saves time and promotes 

 accuracy, as in the usual form (with only one web movable) it is almost 

 impossible, by means of the mechanical stage, to bring an object into exact 

 contact with the fixed web, which is done at once with case and certainty by the 

 second movable one. Having now used tliis a good deal, I certainly prefer 

 it to any other plan ; whatever arrangement is adopted, however, it is neces- 

 sary to determine the value of the scale of the eye-piece with a stage micro- 

 meter, as the least variation in the conditions of the instrument, as e.g. 

 slightly turning the screw collar of the objective, appreciably alters their 

 relations. 



