of the Fishery hoard for Scotland. 19 



The death of the eggs was not due, in my opinion, to any great extent to 

 deficiency in the spawn itself. There are no doubt the risks of non- 

 fertilization and slight injury which the eggs may receive when the spawn is 

 being pressed from the fish. If the roe is quite ripe the pressure required to 

 expel it is very slight. The extent to which these contingencies afiect the 

 eggs is not known. 



Very few of the eggs escaped fertilization, if the swelling-out of the zona 

 to form a peri-vitelline space is to be regarded as a sign that fertilization has 

 taken place. 



It is evident, however, that if there is only one micropyle by which the 

 sperm can enter the egg, the mode in which the eggs are obtained from the 

 fish would almost of necessity preclude a quantity of the eggs from being 

 fertilized. The eggs flow out in a continuous ribbon, i.e., each egg is in 

 contact with an egg before and with one behind. They fall immediately on 

 to a glass plate and adhere to it. It does not seem likely that the sperm will be 

 able to penetrate the egg at any of its planes of contact with other eggs, or 

 with the glass. It would follow, then, that there are several points at which 

 the sperm may enter, or the eggs run a risk of not being fertilized. According 

 to Jousset de Bellesrae, " the robustness of an organism would depend in 

 great measure on the number of fecundating elements which entered the egg." 



In my previous paper I referred to the presence of crystals inside the zona, 

 of the cooled eggs especially. I regarded these crystals as indicating an 

 insufficient aeration. They are evidently excretory products which are not 

 being properly expelled. 



The crystals were prevalent during the present experiment, but they were 

 not found in all the cooled eggs. 



The death of the eggs may perhaps be ascribed in part to deficient aeration. 



The uncooled eggs were in a large tank containing a sufficient supply of 

 water. But the crowded condition of the eggs probably required some sort 

 of strenuous movement of the plates in order to secure a continual change of 

 the water in contact with the ova. 



In the case of the cooled eggs, however, I think that neither the amount 

 of running water nor the movement of the revolving frames was much at 

 fault. I have no doubt that a larger current of water, and possibly a quicker 

 movement, would bo an advantage from the point of view of aeration. Death 

 there, however, was, in my opinion, mainly due to too low a temperature. 

 The development of the eggs was apparently arrested for a time. 



May a slightly deficient aeration inhibit development without killing the egg? 



It is probably the case that as the embryo grows an increased aeration 

 may become necessary. 



Recommendations. 



I think that it would be advisable to get the eggs, as soon as possible after 

 spawning, into running water, or water kept in motion. The eggs are so 

 crowded together on the plates that injury is almost certain to result from 

 leaving the water quiescent. 



During the period of incubation I believe a mechanical aeration is necessary, 

 unless a strong current of water be available. Where the quantity of w^ater 

 is limited, as in the case of cooled water, mechanical aeration seems necessary. 



There appears to be a risk of retarding the eggs too much by exposing them 

 to a very low temperature. 



The tank in which the eggs are kept should not be in direct contact with 

 ice. 



The Spawn. 



The methods adopted for obtaining the eggs of the herring were similar 

 to those of February 1908. I shall give a short description of each lot of 

 eggs, and shall indicate their treatment and history. 



