HATCHING THE EGGS. 139 



as to which eggs are impregnated and which are not, 

 before attempting to decide with much exactness on 

 the percentage of impregnation. 



As remarked above, a fine dark line near the mid- 

 dle of the impregnated egg will be observed, on close 

 examination, about the end of the first third of the 

 hatching period. Soon the whole form of the fish will 

 become cloudily apparent, and then the black eye- 

 spots will appear, first one and then both. Now is the 

 best time to tell what proportion of the eggs are im- 

 pregnated. You can form some estimate, perhaps, be- 

 fore, by taking out a few in the phial, say ten, and 

 counting the impregnated ones in it. If, for instance, 

 nine are visible, then you infer that ninety per cent are 

 good. But this method is very deceptive, and can- 

 not be relied upon, both because the number is too 

 small to base an estimate on, and also because the 

 specific gravity of the empty ones being a little less 

 than that of the full ones, it sometimes happens that 

 a twirl of the feather will throw the empty ones 

 together in a hole, and the impregnated ones together 

 in another pile, on the mechanical principle which 

 leaves sand, marl, and vegetable matter in a brook in 

 different spots by themselves. In taking out three or 

 four in a phial for examination, you may happen to hit 

 upon one of these piles or the other, and so get a 

 deceptive sample of the eggs in general. 



The best way to get the ratio of the good to the 

 worthless ones is to take out several hundreds or a 

 thousand after the eye-spots show plainly, and pick 

 out the empty ones. Count both, and add its proper- 



