POLYVITELLINY IN POND SNAILS. 319 



genetic experiment, the control and other isolated stock which was 

 used in determining the viability of eggs laid by virgins (Crabb, 

 '270) furnish enough experimental data to show that the laying of 

 polyvitelline eggs is not hereditary. Although the genetic ex- 

 periment was discontinued, the occurrence of these abnormal eggs 

 was noted in the records of other experiments. 



MATERIAL AND METHODS. 



Eggs of PJi\sa say ii, Lynunca stagnalis appressa, L. columella 

 and L. palustris were used in this investigation. Egg masses of 

 L. s. appressa and L. palustris containing polyvitelline eggs were 

 placed in 5 per cent, formaldehyde for several hours, or even 

 months; then the abnormal eggs were removed from the mass 

 and mounted in hollowed slides, in 2.5 per cent, formaldehyde, 

 to facilitate studying and drawing the vitelli. After a few weeks 

 in the stronger solution the eggs ceased to shrink appreciably and 

 began to darken but are still transparent after having been in the 

 fluid eighteen months. The albumen of the eggs fixed in the 

 strong solution over night and mounted in the weaker solution has 

 neither coagulated nor blackened, and the vitelli are as plump and 

 bright as they were when the eggs were sealed up over two years 

 ago (Fgs. 3, 12). 



In every instance the eggs recorded for individuals of each 

 species represent all that that snail, or group of snails, laid during 

 the period of observation. Thus, so far as the observations go, 

 the data on the occurrence of polyvitelline eggs is much more re- 

 liable than if the masses had been collected at random. 



NORMAL AND POLYVITELLINE EGGS. 



Normal eggs of the snails herein described consist essentially of 

 an outer membrane enveloping a mass of albuminous substance 

 which surrounds a relatively small, usually excentric, vitellus or 

 yolk. Thus the structure of a pond snail's egg superficially re- 

 sembles that of a bird. 



Morphologically polyvitelline eggs of L. s. appressa and L. 

 palustris differ from normal eggs only as regards the number of 

 vitelli, and the amount of albumen displaced by the supernumerary 

 vitelli. There is as much variation in size between normal eggs 



