210 ARTIFICIAL PARTHENOGENESIS AND FERTILIZATION 
concerned) that this formation of droplets depends upon the fact 
that a colloidal substance, which lies below the surface layer 
of the unfertilized egg or is secreted from the egg, suddenly swells 
by absorption of sea-water. In the typical case of membrane 
formation this swelling results finally in a complete liquefaction 
of the colloid. In other cases the swelling is less complete and 
the formation of a gelatinous film results. 
4. If this idea is correct it should be possible to prove that 
the agencies which cause membrane formation may also cause a 
swelling and liquefaction of some colloidal substance associated 
with the egg. This proof can be furnished in the case of the 
chorion which surrounds the immature egg of Lotta, a molluse 
of the Pacific coast. 
Fig. 63 represents the unripe egg of Lottia. While in this 
condition, the egg cannot be fertilized by a spermatozoon. 
It possesses an irregular outline, owing to the fact that it is 
surrounded by a stout membrane, the so-called chorion. When 
this membrane is removed, the egg assumes the form of a sphere 
(Fig. 65). We will now show that the various substances which 
cause membrane formation in the sea-urchin egg also cause a 
swelling and liquefaction of this chorion. Saponin is, as we 
have seen, one of these substances. If an unripe egg of Lottia 
is placed in 5 c.c. of sea-water to which have been added about 
six drops of a } of 1 per cent solution of saponin (in sea-water), 
in some four minutes it changes from the condition shown in 
Fig. 63 to that in Fig. 64, and in about another four minutes 
the chorion has quite disappeared and the egg has become 
spherical (Fig. 65). A comparison of Figs. 65 and 64 will show 
that under the influence of saponin the chorion first swells 
greatly by the imbibition of sea-water, and finally liquefies. 
A second agency calling forth membrane formation in the 
sea-urchin egg is bases in the presence of free oxygen. As long 
as the unfertilized eggs of Lottia lie in normal sea-water the 
chorion remains unaltered. But if the alkalinity of the sea- 
