SECT. 5] IN ONTOGENESIS 793 



e.g. Stockard's magnesium (cyclopean) embryos. Roffo & Correa 

 studied the osmotic properties of the egg-membrane of the mollusc 

 Valuta brasiliense, and found that they showed no changes during the 

 development of the embryo. 



5-4. The Osmotic Pressure of Fish Eggs 



A different state of affairs came to light, however, when fish eggs 

 were investigated. Runnstrom, for instance, examined the eggs of 

 Salmo by counting the number which developed normally in different 

 solutions. Nordgaard had previously found that these eggs would 

 not develop in sea water, or in 20 per cent, salt solution, though, if 

 fertilisation had been done in fresh water, they could stand 9 per 

 cent. Runnstrom and Svetlov measured the depression of the 

 freezing-point of the egg-contents (excluding the perivitelline liquid) 

 oi^ Salmo savelinus a.nd Salmo fario respectively, obtaining the following 

 results : 



Evidently the changes were very small. The initial slight decrease 

 can be accounted for, according to these workers, by an absorption 

 of water from the hypotonic (fresh water) medium and an increase 

 in the amount of perivitelline liquid, for after 4 hours in water there 

 is a 6 per cent, increase in weight, just as, in the case of Salmo trutta, 

 Miescher found an increase of weight of 10-83 per cent, in the same 

 circumstances, and Bogucki an increase of 20 per cent, volume. In 

 Osmerus eperlams, again, there was no fall or rise of osmotic pressure. 

 Thus the eggs of fresh- water teleosteans are practically independent of 

 the hypotonic medium in which they live, and the same independence 

 is shown by marine teleosteans with regard to their strongly hyper- 

 tonic medium, as will be shown below. On the other hand, as 

 Nordgaard's investigations showed, the fresh-water teleost egg can- 

 not be said to be indifferent to hypertonic solutions. Development 

 in 8-5 per cent, salt solution would be development in a practically 

 isotonic solution, and, as we have seen, this is as much as the Salmo 



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