258 Experimental Zoology 



their early development until the embryo hatches and begins to 

 feed for itself. It appears, however, in the case of some water 

 animals that the egg or the early embryo derives some of the 

 materials necessary for development from the surrounding me- 

 dium. In the development of the eggs of the lower marine 

 animals it has been shown that certain inorganic constituents of 

 the sea water enter directly into the embryo and take part in its 

 growth. The most thorough study of this sort that has been 

 carried out is that of Herbst. His final conclusion is that for 

 normal growth all of the common constituents of sea water are 

 necessary. For a few of them other closely related chemical 

 compounds may be substituted, but it is surprising how little 

 substitution of this sort is possible without altering the normal 

 processes of growth. In such cases it must not be overlooked 

 that the egg itself is a storehouse of food materials, and that the 

 growth consists mainly in adding water to the materials already 

 present; but there can be no doubt that extraneous salts are 

 also fixed by the tissues of these organisms and take a part 

 in the growth. For example, the young embryo of the sea 

 urchin produces a calcareous skeleton. Unless calcareous salts 

 are in the water, the skeleton is not formed ; even other closely 

 related salts cannot be substituted. Quite recently Maas has 

 carried out an experiment with sponges. He finds in them also 

 that unless the calcareous constituents of the sea water are pres- 

 ent, no spicules are formed, and the development does not extend 

 beyond a certain point. 



By adding salt to fresh water the density may be increased. 

 By diluting sea w^ater its density may be decreased. Both 

 changes may aftect the rate of growth, as shown by the following 

 experiments. Yung (1885) added 2.0, 4.0, 6.0, 8.0 grams of sea 

 salt to 1000 grams of fresh water, and placed frog embryos in 

 the solutions. In the .2 per cent solution the rate was nearly 

 the same as in pure water. Retarded development occurred in 

 all the others. In the .8 per cent solutions the tadpoles hatched 

 17 days after the normal time. 



It is not altogether clear in this case whether the effects are 



