III. 7 OSMOTIC PRESSURE 121 



produce the same, but markedly different effects. Some solutions 

 arrest development at an early stage (during segmentation 

 (Fig. 64), gastrulation, or the formation of the medullary folds) ; 

 in others development proceeds but is distorted, the medullary 

 folds remaining open in whole or in part, and the yolk-plug un- 

 covered, or either of these malformations may occur without the 

 other; in one case (dextrose) development is quite normal in 

 form but very considerably retarded, while finally in urea de- 

 velopment is normal both in form and rate (Figs. 66, 67). No 

 legitimate deductions can be made from these experiments, how- 



FIG. 65. Frog embryos grown in a -625% solution of sodium chloride. 

 A and B after five days, c and D after six days. In all the yolk-plug is 

 fully exposed. In A the medullary groove is wholly open, in B and c it 

 is closed behind, in D it is closed throughout. 



ever, until the permeabilities of the tissues to these solutions are 

 ascertained. The tadpole requires water (Davenport), and the 

 degree of shrinkage of the tadpoles in these solutions affords a 

 means of determining the question ; it appears that they are per- 

 fectly permeable to urea, more or less impermeable to cane-sugar, 

 dextrose, and sodium chloride, the shrinkage being rather greater 

 in the first than in the other two. On the assumption that the 

 permeabilities of the embryo are the same as those of the tadpole, 

 it follows that the greater effect produced on the former by sodium 

 chloride than by cane-sugar, or, still more, than by dextrose, 

 cannot be set down to the osmotic pressure of the solution alone, 



