SECT. 5] IN ONTOGENESIS 779 



the neural groove nor hatching affected it, and the rise continued 

 without change until the osmotic pressure of adult serum was 

 attained on or about the 25th day. These time relations held for 

 Rana temporaria, but they were found to be applicable with varia- 

 tions to the eggs of many other amphibia. Backmann & Runnstrom 

 found that parthenogenesis by hypertonic solutions induced at any 

 rate the first of these changes, none of which, indeed, took place 

 in unfertilised eggs. Micrometer measurements demonstrated that 

 during the first few days exceedingly slight changes take place in 

 egg-volume, quite insufficient in magnitude to account for the fall 

 by simple dilution. E.g. : 



% increase in 

 diameter 

 64-cell stage->blastula ... 0-5 



Blastula-^closure of blastopore 0-4 



Closure^" 1 2 hours after closure 4-9 



Diameter of unfertilised egg 3'537 mm.; fertilised 3*79 mm. 



During the later ascent of the osmotic pressure cur\'e, there was 

 a certain growth in volume, but Backmann & Runnstrom only gave 

 a few fragmentary figures for this. The chorionic or perivitelline 

 liquid seemed to be hypotonic to the 5-day old embryo, but hypertonic 

 to the pool water, i.e. about — 0-15°. Backmann & Runnstrom 

 regarded the change in the osmotic pressure of the egg as not of any 

 recapitulatory significance, because some experiments which they 

 made on land frogs gave the same results. Although the eggs normally 

 developed on dry land, they showed the same osmotic pressure 

 changes. They were more inclined to see in these effects an adapta- 

 tion mechanism which had been retained by the land frogs although 

 of no further use. 



Backmann & Sundberg next confirmed de Varigny, who had 

 stated in 1888 that solutions of different osmotic pressure were isotonic 

 with frog's eggs at different stages of their development, without 

 giving any experimental figures^. They also compared their gradually 

 rising curve for osmotic pressure during larval life with the figures 

 for water-content previously found by Davenport and Schaper. The 

 result is shown in Fig. 178. There was undoubtedly a rise in water- 

 content, which came to a level plateau at about 94 per cent, by the 



1 It must be remembered throughout this section that experiments of the type of 

 de Varigny's, unlike freezing-point measurements, only give the osmotic pressure of the 

 substances within the egg to which the membrane is impermeable. 



