MIDDLETHIRD HATCHING-HOUSE. 119 



tributing the current so as to flow evenly in the plane of the ova. 

 Danger from sediment renders it advisable to leave some space 

 between the bottom of the tray and the ova, and experience has 

 shown the best results are obtained with not more than an inch 

 of water over the eggs. At least l inch is required below the 

 ova, so that the current may be increased just before hatching 

 without disturbing the sediment at the bottom ; there is always 

 some deposit, even from the clearest spring- water. The width of the 

 box is important. It is much more difficult in practice to ensure 

 an even flow where the width is great than where it is narrow, 

 and the Howietoun hatching-box is now divided by a moveable 

 board ; but at Middlethird I did not require to deal with large 

 numbers, at least not at the time of which I am now writing. The 

 width was fixed solely with the view of keeping the current under 

 control, and 10 inches inside measurement was thought suitable. 

 I afterwards found another reason for this width when I made the 

 glass grilles. The hollow tubes of glass are pliant to a degree, and if 

 more than 9 inches long that is, 8^- between the fastenings they 

 bend out in the middle, and allow the ova to slip through. The 

 width being determined, I considered whether a slight fall on the 

 box would not be of service ; and the first season I fixed the 

 boxes with a fall of a quarter of an inch to the foot. This did not 

 answer, as, when the water was turned off for cleaning the filter, 

 the eggs nearest the top of the box were left dry. I then tried 

 heightening the water in the box, by placing flannel on the outlet. 

 This was worse ; the eggs at the lower end of the box were too 

 deep : they got too little oxygen, the current being modified by 

 the depth of the water. I next set the boxes level, and placed 

 cleats across the bottom, which answers perfectly, and are so used 

 in the Howietoun hatching-boxes at present. Having at last 

 achieved an even flow, the eggs are supplied with oxygen by the 

 water, and the gases given off in the process of incubation 

 removed. The amount of gas given off is great, and, if not 

 removed would suffocate the embryo, especially just before hatch- 

 ing. Place a dozen or two eyed ova in a dish, and leave them for 

 a couple of days in a room, the windows of which are kept closed, 



