SECT. 5] IN ONTOGENESIS 799 



In 1928 Sumwalt pointed out that the Fundulus embryo was 

 enclosed in two membranes, in the early stages, by the outer 

 chorion or egg-membrane and the vitelline membrane, and, in the 

 later stages, by the chorion and the skin. The importance of the skin 

 factor in the ontogenesis of water regulation, etc., had already been 

 emphasised in the case of amphibia by Adolph, but Sumwalt used 

 a new method, measuring the permeability to ions of the various 

 embryonic membranes o^ Fundulus in terms of concentration potentials 

 across the membranes between solutions of jV/io and JV/ioo potassium 

 chloride after Michaelis. By means of a capillary pipette in a micro- 

 manipulator, one electrode was introduced into the inside of the egg, 

 and a similar electrode dipped into the solution in which the egg 

 was placed. For a measurement of concentration potential through 

 the chorion alone, the electrode was placed in the sub-chorionic or 

 peri vitelline space; for a measurement across the chorion plus the 

 embryonic skin, it was put in the yolk-sac of the embryo. The results 

 showed that the compound membrane of skin plus chorion could 

 produce much greater concentration potentials than the chorion 

 alone, the average for the latter being 19-4 millivolts, and for the 

 former 55-2. In both cases change to the dilute solution (sea water 

 diluted 100 times) caused the inside of the egg to become negative 

 to the outside, indicating a relatively greater impermeability of the 

 membranes to anions than to cations. This is less pronounced in the 

 chorion than in the skin. Measurements of electrical resistance con- 

 firmed this view, for the resistance of the chorion alone was about 

 45,000 ohms, but that of the embryonic skin about 208,000. 



5-5. Osmotic Pressure and Electrical Conductivity in Worm 

 and Echinoderm Eggs 



Of the osmotic pressure of oligochaete worm eggs very little is 

 known, but there is in this connection an interesting study by Svetlov 

 of the eggs of the Lumbricidae, Bimastus constrictus, and Eiseniafoetida. 

 The Terricolae, the sub-order to which the earthworms belong, lay 

 their eggs in cocoons, which were formerly mistaken for the eggs 

 themselves. These cocoons are brown and horny and vary in size 

 according to the species; they contain ova and spermatozoa as well 

 as a milky nutritive fluid in which the young worms float and by 

 which they are nourished before they hatch out from the cocoons. 

 Svetlov had morphological and cytological reasons for supposing 



