OCCUKRING IN THE NEIGHBOURHOOD OF PLYMOUTH. 31 



and found tliat in eleven years, in March three times,, in April three 

 times, in May seven times, the maximum specific gravity of the 

 water off Kiel Bay was lower than that of the plaice ovum. Thus 

 in these years the ova of Pleuronectes species would have to develop 

 on or near the bottom, Hensen does not give any details of the 

 apparatus in which he hatched the ova artificially, but I infer from 

 his remai-ks that he hatched them when they sank in the water of 

 the vessels containing them. 



But of course it does not follow that mackerel ova will develop 

 healthily in water of less specific gravity than themselves. And it 

 is certain that it was a constant result in my experiments that 

 mackerel ova floating at the surface in still water, even when the 

 density was artificially increased with common salt, lived some days, 

 while those provided with a circulation of water in which the ova 

 sank died in a much shorter time. The same thing was observed 

 also with the ova of Pleuronectes microcephaliis and others. But 

 these experiments are not sufficiently rigid to prove that the too 

 low density of the water, i. e. the sinking of the ova, was the sole 

 cause of the death of the ova in unsuccessful experiments. It may, 

 however, be pointed out that in my experiments the ova were first 

 fertilized in water much denser than themselves, and remained in 

 this some time before they were transferred to water of less specific 

 gravity in which they sank, and a change of density like this after 

 fertilization may possibly be fatal, when if the ova were shed and 

 fertilized at the beginning in water in which they sank they would 

 live and develop normally. This can only be decided by further 

 experiments directed specially to this point. 



The specific gravity of mackerel ova fertilized in the water of 

 the open sea where the parents were captured is 1'0265 at the 

 maximum. In water of a specific gravity of 1*0263 a few out of a 

 large number of ova remained at the surface ; at a specific gravity of 

 1*026 the ova remained suspended at various depths for some time, 

 and one or two rose to the surface, while at l'02o7 all the ova sank 

 rapidly. Thus the specific gravity of different individual ova varies 

 slightly within narrow limits ; a few were observed to have a specific 

 gravity of less than 1*026, but the great majority are heavier than 

 1*0263, and none are heavier, at least at early stages of development, 

 than 1*0265, or lighter than 10259. 



Development. — The ovum of the mackerel after fertilization is 

 spherical and transparent, and has a diameter of 1*22 mm. In the 

 early stages of segmentation the ovum is not perfectly spherical, 

 because the principal diameter, passing through the centre of the blas- 

 toderm, is a little longer than the others, owing to the slight pressure 

 of the prominent blastoderm on the envelope (fig. 16). The yolk is 



