III. 7 



OSMOTIC PRESSURE 



119 



increased osmotic pressure of the medium and consequent with- 

 drawal of water from, or prevention of imbibition of water by, 

 the eggs, the weak point of the experiment, and of all such 

 experiments, is our ignorance of the extent to which the ova or 

 embryos are permeable to the substance employed, since the 

 osmotic effect, or withdrawal of water, will obviously vary in- 

 versely with the permeability. The neglect of this possibly 

 disturbing factor has indeed led in some cases to quite un- 

 warrantable conclusions. 



In 1895 O. Hertwig showed that certain abnormalities could 

 be produced by growing the eggs of the Frog (If. fnsca and 

 A Be 



FIG. 62. Three sodium-chloride embryos of Rana fusca. df, yolk- 

 plug ; hp, brain ; ki, gills ; s, margin of epidermic layer of ectoderm ; 

 sch, tail ; ur, lip of blastopore. (After 0. Hertwig, from Korschelt and 

 Heider.) 



esculenta] and of the Axolotl in a solution of common salt. In 

 stronger solutions (1 % to 0-8 %) segmentation was confined to the 

 animal hemisphere, though nuclear division went on in the 

 yolk. Weaker solutions (0-6%) allowed of further, but dis- 

 torted, development; the yolk-cells were unable to move 

 beneath the lip of the blastopore, so that the latter remained 

 open with a persistent yolk-plug, and the medullary folds failed 

 to close in the region of the brain, a condition recalling the 

 abnormalities known in Human and Comparative Teratology 

 as Hemicrania and Anencephaly (Figs. 62, 65). The exposed 

 region of the brain underwent a grey degeneration with dis- 



