530 Phylum Mollusca 



September or the first of October, and the laying season lasts about six 

 months. The number of eggs at a single laying varies widely, being 

 from 3 to 48 in this laboratory. The usual number in a batch, however, 

 is about two dozen. The number depends upon the size of the slug, 

 the season, and the care that the animal has had. If they are allowed 

 to lay their eggs in the stock jars, they often make a depression in the 

 leaf mold and deposit the eggs in it. 



The eggs are fastened together by a membrane and thus all in a 

 batch are laid at once. The slugs do not move about while laying; 

 hence the eggs become superimposed and are able to retain their mois- 

 ture for a longer period than a single egg. The moist leaf mold serves 

 to keep the eggs from being desiccated but some eggs have hatched 

 after losing from 80% to 85% of their weight; or 70% to 75% in 

 the case of eggs containing full-term embryos. 



The eggs are elliptical in shape, the average diameter being about 

 0.5 cm. and the average length being about 0.75 cm. The weight of 

 the individual eggs in a batch is fairly constant, but eggs from dif- 

 ferent batches have varied from 39 to 95 mg. each. They are cov- 

 ered with a rather tough membrane which permits them to be handled 

 with considerable ease when they are being removed from the leaf 

 mold. 



For washing a batch of eggs it has been found convenient to put 

 them in some water in a small beaker or petri dish for a few minutes 

 and then remove them by means of a spatula to a dry piece of filter 

 paper. Any mucous or dirt from the stock jar is easily removed by this 

 process. 



The time required for hatching at room temperature was found to 

 vary from 19 to 27 days but a majority of the fertilized batches began 

 to hatch by the 23rd day. Most of the batches were fertile in the fall 

 but the fertility decreased within a few weeks, being much reduced by 

 January and it was often quite low by the last of February. Occasion- 

 ally a whole batch did not contain a single fertilized egg. Since the 

 eggs are transparent it is possible at an early stage to observe the 

 number of embryos that are developing. It is also possible to study the 

 rate of growth. Several eggs were noted to contain twins, and these 

 usually hatched. In the latter part of the laying season single eggs 

 sometimes contained as many as nine embryos. They did not live 

 long enough to hatch when an egg contained three or more. 



After the embryos hatched they were placed, by means of a spatula, 

 upon moist filter paper in a clean petri dish. Deeply pigmented embryos 

 are especially sensitive to ultra-violet rays. For this reason and because 

 the slugs are nocturnal animals, it is best to keep the eggs and young 

 in a dark place. 



