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 Lottia, a mollusc 

 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 J 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- 



