of the Fishery Board for Scotland. 



139 



(fresh) were weighed. Knowing the weight of each ovum, and 

 having calculated the volume of each, the specific weight, of course, 

 is easily obtained. 



This does not give good results. In the first place, because the 

 eggs, in most cases, are not complete spheres ; and, secondly, in the 

 measurement they become flattened out as they lie on the slide. 

 However many you calculate the diameter of, you are not in a 

 position to accurately determine the volume. 



(3.) The method of taking the specific gravity by floating the 

 eggs in a series of vessels containing NaCl. CI. solution of diflerent 

 specific gravities was employed. A large number of vessels was taken, 

 and each was filled with salt solution of fixed specific gravity (this was 

 noted down on a label affixed to jar). Then ova were dropped into each 

 of these dishes, and those two vessels in which the ova could just 

 float and just sank were noted, and between those two a further 

 series of observations was made by dropping in more ova. The 

 specific gravity of the fluid, which could just support the ova, was 

 taken as the correct one for the ova. This is a good method, only 

 is open to at least one fallacy — viz., that as the ova lie in the fluid 

 osmosis takes place between salt solution and contents of ova ; and 

 hence, if some time elapse between the putting in of the ova and 

 the determination of their specific gravity, some gradually sink, or 

 some which have sunk may rise. In the extremely interesting case 

 of the angler fish (Lophius), it is not the specific gravity of the 

 ova per se that is of importance, but that of the ova imbedded in 

 the jelly-like interstitial matrix, as it is in that form that the roe is 

 spawned. 



(4.) The fourth method is one which may be termed estimation 

 of specific gravity by displacement. It is only practicable with 

 bodies that are insoluble in water. The ova are certainly not soluble, 

 at least to any appreciable extent, in water during the short time in 

 which the determination is being made. 



It is carried out in the following way : — A piknometer is weighed 

 filled with distilled water ; then a quantity of the ova, sufficient to 

 fill the greater part of the piknometer, is weighed between a pair of 

 well-fitting weighed watch-glasses connected by a clip, the weight 

 being carefully noted. Now all these ova are carefully dropped 

 into the piknometer, when they fall to the bottom and displace an 

 equal volume of water. The stopper is replaced on the flask, and 

 the surface dried. Now the piknometer is weighed again. The 

 specific gravity is, of course, now easily estimated, thus : — If w = wt. 

 of ova, W = wt. of pik. + water, = wt. of pik. + water (not dis- 



w 



placed by ova) + w, then Sp. Gr. = — ■ 



W + w - 



This, I think, is the best method of the four described, and most 

 of the estimations have been done by it; in fact, all have been 

 controlled in this way. 



B. — Calculation of Diameter of Ova. 



This was done by means of a Zeiss ocular micrometer, the value of the 

 divisions in it being fixed by means of a stage micrometer. An average 

 was taken for a large number of ova. 



