MACKEREL INVESTIGATIONS IN 1897. 17 



But suppose tliey do normally sink in the 20 oi- 30 fathoms of water 

 found where they were collected, or in the still deeper water whence 

 most of tliem in all probability came, what conditions do they meet? 



First, a column of water many (several hundred) times as high as 

 that in the boxes and Jars in which artificial i)ropagation has been 

 attempted. For a body of but little lower specific gravity to pass 

 through such a stratum of water would require, even were the water in 

 a perfectly quiescent state, a very considerable length of time — per- 

 haps (]uite suliHcient to permit hatching before the bottom is reached. 

 But the constant slight movement to which these waters are subject 

 owing to their agitation by tide, winds, and constant currents would 

 greatly prolong the time of descent. 



Secondly, the density undergoes a constant and in deep water a 

 frequently considerable increase toward the bottom, so that the egg in 

 its progress downward would always pass through water of a density 

 corresponding to its own increasing specific gravity, and would thus 

 be constantly buoyed np by a nicely adjusted force, the result of which 

 would be to further delay it. 



It is probably safe to conclude, then, that under natural conditions 

 the egg of the mackerel is always suspended in water of a density 

 very nearly equal to or greater than its own, and that it does not 

 normally settle to the bottom and rest there in a mass of filth, and sub- 

 jected to conditions of imperfect oxygenation, etc., which is the case 

 in the forms of artificial hatching apparatus, namely, the Chester and 

 McDonald systems of tidal boxes which have hitherto been chiefly 

 employed. Another consideration to be noted is that the parallel 

 increase in density of the water and the specific gravity of the egg will 

 result in the maintenance of a nice balance of osmotic pressure which 

 may possibly be a requisite to a healthful development. Three other 

 conditions of change relate to light, oxygenation, and temperature, all 

 of which decrease toward the bottom. 



3. RESULTS OF EXPERIMENTS. 



When the results of biological investigations of the conditions of 

 development were sufficiently advanced to have led to the conclusion 

 just briefly outlined, several important lines of experimentation seemed 

 to be indicated. Direct observation of the changes in the living egg 

 and of the conditions aflecting the distribution of the egg in nature 

 having suggested the importance of an increasing water density, a 

 series of experiments designed to test the influence of this condition 

 was first planned. Indeed, owing to the paucity of material and the 

 limited time in which to work, this series was the only one which was 

 conducted in anything like a systematic manner, and even here the 

 results of individual experiments, which could not be repeated and 

 verified, are too meager to be conclusive. 



A description of one or two of these experiments, with their results, 

 will suffice to indicate the general character and bearing of all. 

 F. R. 98 2 



