xi PRACTICAL HINTS ELASMOBRANCHII 561 



For hatching boxes ii is convenient to take ordinary fish 

 I >o.\rs freely perforated with auger holes, provided with a cross 

 partition in the centre, and pitched inside, and out to discourage i In- 

 growth of seaweeds. The hatching boxes are moored afloat in pure 

 sea-water within a breakwater or other shelter. About 20 eggs are 

 placed in each compartment. 



< >M alternate days the boxes are drawn a few times backwards 

 and forwards through the water to dislodge any sediment that may 

 have accumulated. Once a week they are hauled out of the water 

 and eacli e^-shell tested by rubbing the finger over its surface. If 

 a slippery mucus-like layer has developed on its surface the egg is 

 useless and should be got rid of. 



When the egg has reached the desired period of development it 

 is removed from the water, placed in a horizontal position with the 

 more strongly convex side below and opened by carefully removing 

 the greater part of the less convex side of the shell. The isolated 

 piece of shell must be lifted oft' very carefully as the albumen is very 

 adhesive and the vitelline membrane extremely delicate. 



In the early stages the embryo is almost invisible in the fresh 

 state so the egg, still held carefully in a horizontal position, is gently 

 submerged in fixing fluid. The blastoderm then comes into view 

 and after a short time may be excised and floated into a watch-glass 

 to complete fixation and the subsequent processes. 



In later stages (Fig. 248) where the body of the embryo is 

 constricted off from the yolk-sac, it is narcotized by 'submersion in 

 sea- water containing 3% alcohol and then the yolk-stalk is ligatured 

 with thread and the embryo excised for further treatment. 



Embryological material of the Sharks is to be preferred to that of 

 the Skates or Kays on account of their, less specialized character but 

 mi fortunately it is more difficult to obtain in quantity. Small 

 sharks of the genus Scyllium and allied genera occur commonly round 

 the shores of the various continents and their eggs may be found 

 attached to seaweed at extreme low tides. 



On the British coasts a well-known spawning ground for Scyllium 

 exists at Careg Dion about 2J miles from Beaumaris on the 

 Anglesea side of the Menai Straits in between 3 and 4 fathoms of 

 water and in spots not exposed to strong tidal currents. 1 The eggs 

 are deposited usually in the morning, the shorter stouter pair of 

 filaments which issue first from the cloacal opening being trailed 

 about amongst tufts of the seaweed Halidrys siliquosa until they 

 become entangled when the fish swims round so as to wind the 

 elastic filaments firmly amongst the seaweed. The eggs can only 

 be obtained at very low and specially favourable spring tides and as 

 White finds at one time embryos of all stages of development it would 

 appear that oviposition is not limited to any definite season. 



Scyllium not infrequently deposits its eggs in aquaria and at the 



1 For thr details in regard to this locality I have to thank Professor -Philip J. 

 White of 



VOL. II 2 



