EGG CELLS DEVELOPING UNFERTILISED 217 



going, namely, by a mere prick of the surface film of the 

 ripe egg ! 



There have, however, been important experiments on 

 the subject of the development of eggs without fertilisa- 

 tion in recent years, prior to these discoveries as to the frog's 

 egg. A favourite subject for such inquiries is the sea 

 urchin (Echinus of different kinds). The female sea 

 urchin, or sea egg, like its close allies the star fishes, lays 

 a great number of very transparent minute eggs (each 

 about the -g-Jirth f an mcn ' m diameter) in sea-water, and 

 they are there fertilised by the mobile sperm-filaments 

 discharged by the males. The eggs are so transparent 

 and so easily kept alive in jars of sea- water that there is 

 no difficulty in watching under the microscope the pene- 

 tration of the egg by a sperm, and the fusion and other 

 changes in the nuclei. Delages of Paris, and Loeb of 

 California, have made valuable studies on these eggs. 

 Loeb has shown that they may be artificially started on 

 the course of development and cell division without fertili- 

 sation simply by the action of minute quantities of 

 simple chemicals (fatty acids, etc.) introduced into the sea- 

 water by the experimenter. These chemicals appear to 

 act on the delicate pellicle which forms the surface of the 

 egg-cell in much the same way as the prick of a needle 

 acts on a frog's egg. A limited and delicately adjusted 

 disturbance of the cohesion (or of the surface-tension) of 

 the egg-cell seems to be all that is necessary for starting 

 the egg-cell on its career of development. It becomes, in 

 the light of these experiments, not so much a wonder that 

 egg-cells should develop " on their own," but that they do 

 not more frequently do so. It must be remembered that 

 the " germination " and development of unfertilised eggs, 

 even when the whole range of animals and plants is taken 

 into account (for plants also are reproduced by single cells 

 identical in character with the egg-cells and sperm-cells 



