274 MK. a. BROOK ON THE 



seem to have a case in point of the prevalence of particular 

 species of shells being influenced by M'ave-currents acting on 

 sea-bottoms that do not remain constant in constitution and 

 character. 



Preliminary Account of tbe Development of the Lesser Weever- 

 rish, Trachinus vipera. By GtEORGE Beook, P.L.S. 



(Bead Ist May, 1884.) 



[Plates III.-VI.] 



The observations on which my paper is based have been made on 

 eggs laid in my aquarium by fish vphich I have bad in the tanks 

 over two years. The conditions under whicb the development 

 was carried on will not therefore be normal, and tbe direct rays 

 of the sun were never allowed to fall on the eggs, as would be 

 the case in nature. 



The eggs of Trachinus vipera are laid in tbe summer. I have 

 had them as early as April, both last year and this, as floating 

 eggs. Dead eggs have been found at the bottom of the tanks iu 

 March. The eggs found in April were very few, and often not 

 ! fertilized. It was not till the 6th of June that they began to 

 show in any numbers, and with but a small proportion of unfer- 

 j tilized ones. They continued to be laid at intervals of three or 

 four days during June and July ; but the batches laid during 

 the last few days of July were again few in number, and with a 

 large proportion of unfertilized ova amongst them ; and no ova 

 were found in August at all. 



The egg of Trachinus vipera is about 1"32 millim. in diameter, 

 of a beautiful pearly white, and quite translucent, and contains 

 from 20 to 30 small oil-globules which cause it to floaty on the 

 surface of the water. These oil-globules are scattered over the 

 upper hemisphere of the yolk, and lie between it and the vitel- 

 line membrane. They vary in size from "12 to "03 millim. The 

 oil-globules cause the egg to float with the germinal disk down- 

 wards, so that the embryo is developed on its back, so to speak, 

 and it is not until some time after hatching that the young fish 

 is enabled to swim with the ventral surface downwards. 



Eggs freshly extruded from the ovary are not spherical, as the 

 egg-membranes are larger than the yolk, and appear wrinkled 

 until the "breathing-cavity" gets filled with water, and it is 



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