SECT. 5] 



IN ONTOGENESIS 



82] 



A good deal of our information about the specific gravity of marine 

 eggs is inexact, for it is derived from the reports of those who have 

 studied the frequency of the occurrence of forms of different develop- 

 mental stages at different depths. Thus we know from the work of 

 Holt that the eggs of the turbot Rhombus maximus sink rather quickly 

 after the 7th day of development, and, according to Ehrenbaum, the 

 eggs in the plankton sink as a general rule, for the lower the catch 

 the more advanced the embryos. Raffaele, again, found that nearly 

 all pelagic fish eggs get heavier as development proceeds, presumably 

 because of loss of buoyant oily substances by combustion. Most 

 eggs begin to sink at once, but one (that of Labrax) very slowly, 

 and one {Trachinus vipera) only when development is half completed. 

 Similarly Jespersen & Taning observed that the larvae of the small 

 oceanic fish, Vinciguerria attenuata, move down into deeper water 

 as they develop (see also Yagle). 



Indeed, as far back as 1897 Hensen & Apstein affirmed that the 

 eggs of the cod, Gadus morrhua, sank regularly during development, 

 for at the lower levels only the more advanced stages were found. 

 This was denied by Hjort & Dahl, and by Kramp, but Jacobsen 

 & Johansen (for Gadus and Pleuronectes) and Bowman confirmed it. 

 The most recent investigations, those of Russell, show that eggs of 

 Gadus morrhua and of Sardina pilchardus certainly sink, but those 

 of Clupea sprattus seem to be equally distributed at all depths, and 

 those of Onos are at all stages most abundant just under the surface 

 of the water. Franz in 191 1 supplied some quantitative data as 

 shown in the following table, which demonstrated that at any rate 

 for many eggs the specific gravity was higher at the end of the 

 development than at the beginning. 



Table 93. 



Mackerel {Scomber scomber) 



Gurnard ( Trigla gurnardus) 



Lemon dab (Pleuronectes microcephalus) 



Turbot {Rhombus maximus) 



Sprat {Clupea sprattus) 



Tadpole fish {Raniceps raninus) ... 



Gunner {Ctenolabrus rupestris) 



Rockling {Motella) 



Dragonet {Callionymus lyra) 

 Sole {Solea lutea) ... 

 Dab {Pleuronectes limanea) 



