212 COLLEGE ZOOLOGY 



8. ARTIFICIAL PARTHENOGENESIS 



The eggs of echinoderms pass through a total and equal cleav- 

 age, and are easily fertilized and reared to the larval stage in 

 the laboratory. For these reasons they have become classical 

 material for embryological studies and for experimental purposes. 



One of the most interesting phenomena discovered by means 

 of experiments with echinoderm eggs is the development of a 

 larva from an unfertilized egg when subjected to certain environ- 

 mental conditions. This phenomenon is known as artificial 

 parthenogenesis. The eggs of other animals, for example anne- 

 lids, are also capable of developing under certain conditions 

 without fertilization, and those of some species, like plant lice 

 (Chap. XIII) and rotifers (p. 181), are normally parthenogenetic, 

 but echinoderm eggs have been used for experimental purposes 

 more frequently than any others. 



Loeb reared normal larvae from unfertilized eggs of echino- 

 derms by immersing them in solutions such as chloride of sodium, 

 potassium bromide, cane-sugar, etc. He considered the in- 

 creased osmotic pressure the cause of development, and thought 

 it probable that in ordinary fertilization the spermatozoon brings 

 a solution with a high osmotic pressure into the egg, thereby 

 causing the withdrawal of water. Sea-water concentrated to 

 70 per cent of its volume has a similar result. A lowering of the 

 temperature of sea-water to the freezing-point causes eggs of 

 Asterias and Arbacia to develop; when combined with a chem- 

 ical reagent, a higher per cent of blastulae results. Eggs ex- 

 posed to a higher temperature (35 to 38 C.) during the early 

 maturation period develop parthenogenetically, and even me- 

 chanical agitation may have a similar effect. Normal mitotic 

 figures appear during the cleavage of these eggs. None of the 

 larvae thus produced was reared to the adult stage. 



The ease with which echinoderm eggs can be handled has led 

 to some experiments that have an important bearing upon hered- 

 ity. Of these may be mentioned the fertilization of the eggs of 



